llvm dev - Feb 2020 - [RFC] Allowing debug intrinsics to reference multiple SSA Values

Currently, the debug intrinsic functions each have 3 arguments: an SSA value
representing either the address or Value of a local variable, a DILocalVariable,
and a complex expression. If the SSA value is an Instruction, and that
Instruction is at some point deleted, we attempt to salvage the SSA value by
recreating the instruction within the complex expression. If the instruction
cannot be replicated by a complex expression, then the variable location is
dropped causing a reduction in coverage. One of the key restrictions on this
process at the moment is that each intrinsic can only reference a single SSA
value; a numeric constant may be represented within the expression itself,
allowing for binary operators with a constant operand to be salvaged:

%c = add i32 %a, 4
llvm.dbg.value(metadata i32 %c, DILocalVariable("x"), DIExpression())
; Salvage...
llvm.dbg.value(metadata i32 %a, DILocalVariable("x"),
DIExpression(DW_OP_constu, 4, DW_OP_plus))

This proposal is to allow multiple SSA value references within a debug
intrinsic, allowing binary operators with non-constant operands to be salvaged.
This is a long-awaited feature, with an open bugzilla[0] and support from
members of the community[1][2]. To implement this change, each debug intrinsic
will contain a list of SSA values instead of just one, and a new operator will
be added: DW_OP_LLVM_register, which takes as its only argument the index of an
SSA value within the intrinsic's list, and pushes that SSA value onto the
expression stack. Two proposed syntaxes for the list of SSA values - though
suitable alternatives may be worth considering - are to either replace the first
argument of the intrinsic function with an MDNode containing the SSA values as
operands, or to remove the first argument and make the intrinsic function
variadic, passing the SSA value list as vargs:

%c = add i32 %a, %b
llvm.dbg.value(metadata i32 %c, DILocalVariable("x"), DIExpression())
; Salvage...
llvm.dbg.value(!{metadata i32 %a, metadata i32 %b},
DILocalVariable("x"), DIExpression(DW_OP_LLVM_register, 0,
DW_OP_LLVM_register, 1, DW_OP_plus))
; Alternatively, the intrinsic function could be made variadic...
llvm.dbg.value(DILocalVariable("x"), DIExpression(DW_OP_LLVM_register,
0, DW_OP_LLVM_register, 1, DW_OP_plus), metadata i32 %a, metadata i32 %b)

The new operator DW_OP_LLVM_register would exist in the IR and MIR, and further
down the pipeline be replaced by the appropriate operator for the target debug
output. For example, when producing DWARF this would be replaced by
DW_OP_regval_type, which pushes the contents of a given register interpreted as
a value of a given type onto the DWARF expression stack.

This has the potential to allow salvaging in a much greater number of cases than
is currently possible. There are also potential follow-up tasks, such as
allowing the salvaging of conditional values, that would further improve debug
variable availability. The following table gives, for several of the
multi-source application projects in the test suite, the number of successful
invocations of SalvageDebugInfo, and the number of failed salvages for each type
of unsalvageable instruction:

            Success             Variadic Binops     Variadic GEPs       Cmp
Insts           Select Insts        Load Insts          Phi Nodes          
Alloca Insts        Call Insts
ALAC        261                 29                  61                  0       
0                   1                   12                  0                  
0
Burg        50                  1                   9                   0       
1                   95                  6                   0                  
0
hbd         514                 16                  1                   0       
3                   45                  10                  0                  
4
Lua         270                 12                  54                  0       
12                  46                  32                  1                  
0
minisat     458                 10                  10                  3       
1                   35                  4                   0                  
0
sgefa       439                 1                   121                 0       
20                  14                  55                  0                  
0
SIBsim4     153                 15                  6                   0       
3                   40                  3                   0                  
0
siod        112                 2                   1                   0       
0                   11                  5                   2                  
1
SPASS       1241                70                  27                  27      
27                  2114                156                 0                  
7
spiff       39                  0                   15                  0       
0                   7                   2                   1                  
0
sqlite3     2322                94                  167                 6       
37                  1143                136                 4                  
10
treecc      127                 1                   0                   0       
1                   350                 37                  0                  
1
viterbi     7                   0                   1                   0       
1                   1                   0                   0                  
0


Of these categories, the first 3 will become salvageable after this work is
implemented, and Select Insts will also be salvageable with some follow-up work
to enable conditional branching in complex expressions. The remainder are not
salvageable in general, although it's possible that the specific passes that
delete those instructions may be able to preserve the debug info as long as the
code isn't totally dead.

[0] https://bugs.llvm.org/show_bug.cgi?id=39141
[1] https://reviews.llvm.org/D51976#1237060
[2] http://lists.llvm.org/pipermail/llvm-dev/2019-November/137021.html
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Great! This seems like a crucial feature for building expressions to
represent induction variables, which I am told is an important long
standing problem. From your proposed solutions, I like the idea of making
dbg.value variadic.

In the long run, I think we should consider making llvm.dbg.value a first
class IR instruction. My reasons for that mostly have to do with
efficiency, which doesn't relate to what you are doing, so I deleted what I
wrote on that topic to avoid derailing. I think using a variadic intrinsic
is consistent with this long-term design idea, to be discussed further at
another time.

On Thu, Feb 20, 2020 at 8:55 AM Tozer, Stephen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Currently, the debug intrinsic functions each have 3 arguments: an SSA
> value representing either the address or Value of a local variable, a
> DILocalVariable, and a complex expression. If the SSA value is an
> Instruction, and that Instruction is at some point deleted, we attempt to
> salvage the SSA value by recreating the instruction within the complex
> expression. If the instruction cannot be replicated by a complex
> expression, then the variable location is dropped causing a reduction in
> coverage. One of the key restrictions on this process at the moment is that
> each intrinsic can only reference a single SSA value; a numeric constant
> may be represented within the expression itself, allowing for binary
> operators with a constant operand to be salvaged:
>
>
>
> %c = add i32 %a, 4
>
> llvm.dbg.value(metadata i32 %c, DILocalVariable("x"),
DIExpression())
>
> ; Salvage...
>
> llvm.dbg.value(metadata i32 %a, DILocalVariable("x"),
> DIExpression(DW_OP_constu, 4, DW_OP_plus))
>
>
>
> This proposal is to allow multiple SSA value references within a debug
> intrinsic, allowing binary operators with non-constant operands to be
> salvaged. This is a long-awaited feature, with an open bugzilla[0] and
> support from members of the community[1][2]. To implement this change, each
> debug intrinsic will contain a list of SSA values instead of just one, and
> a new operator will be added: DW_OP_LLVM_register, which takes as its
> only argument the index of an SSA value within the intrinsic’s list, and
> pushes that SSA value onto the expression stack. Two proposed syntaxes for
> the list of SSA values - though suitable alternatives may be worth
> considering - are to either replace the first argument of the intrinsic
> function with an MDNode containing the SSA values as operands, or to remove
> the first argument and make the intrinsic function variadic, passing the
> SSA value list as vargs:
>
>
>
> %c = add i32 %a, %b
>
> llvm.dbg.value(metadata i32 %c, DILocalVariable("x"),
DIExpression())
>
> ; Salvage...
>
> llvm.dbg.value(!{metadata i32 %a, metadata i32 %b},
DILocalVariable("x"),
> DIExpression(DW_OP_LLVM_register, 0, DW_OP_LLVM_register, 1, DW_OP_plus))
>
> ; Alternatively, the intrinsic function could be made variadic...
>
> llvm.dbg.value(DILocalVariable("x"),
DIExpression(DW_OP_LLVM_register, 0,
> DW_OP_LLVM_register, 1, DW_OP_plus), metadata i32 %a, metadata i32 %b)
>
>
>
> The new operator DW_OP_LLVM_register would exist in the IR and MIR, and
> further down the pipeline be replaced by the appropriate operator for the
> target debug output. For example, when producing DWARF this would be
> replaced by DW_OP_regval_type, which pushes the contents of a given
> register interpreted as a value of a given type onto the DWARF expression
> stack.
>
>
>
> This has the potential to allow salvaging in a much greater number of
> cases than is currently possible. There are also potential follow-up tasks,
> such as allowing the salvaging of conditional values, that would further
> improve debug variable availability. The following table gives, for several
> of the multi-source application projects in the test suite, the number of
> successful invocations of SalvageDebugInfo, and the number of failed
> salvages for each type of unsalvageable instruction:
>
>
>
>             Success             Variadic Binops     Variadic GEPs
> Cmp Insts           Select Insts        Load Insts          Phi
> Nodes           Alloca Insts        Call Insts
>
> ALAC        261                 29                  61
> 0                   0                   1
> 12                  0                   0
>
> Burg        50                  1                   9
> 0                   1                   95
>  6                   0                   0
>
> hbd         514                 16                  1
> 0                   3                   45
> 10                  0                   4
>
> Lua         270                 12                  54
> 0                   12                  46
> 32                  1                   0
>
> minisat     458                 10                  10
> 3                   1                   35
>            4                   0                   0
>
> sgefa       439                 1                   121
> 0                   20                  14
> 55                  0                   0
>
> SIBsim4     153                 15                  6
> 0                   3                   40
> 3                   0                   0
>
> siod        112                 2                   1
> 0                   0                   11
> 5                   2                   1
>
> SPASS       1241                70                  27
> 27                  27                  2114
> 156                 0                   7
>
> spiff       39                  0                   15
> 0                   0                   7
> 2                   1                   0
>
> sqlite3     2322                94                  167
> 6                   37                  1143
> 136                 4                   10
>
> treecc      127                 1                   0
> 0                   1                   350
> 37                  0                   1
>
> viterbi     7                   0                   1
> 0                   1                   1
> 0                   0                   0
>
>
>
>
>
> Of these categories, the first 3 will become salvageable after this work
> is implemented, and Select Insts will also be salvageable with some
> follow-up work to enable conditional branching in complex expressions. The
> remainder are not salvageable in general, although it's possible that
the
> specific passes that delete those instructions may be able to preserve the
> debug info as long as the code isn't totally dead.
>
>
>
> [0] https://bugs.llvm.org/show_bug.cgi?id=39141
>
> [1] https://reviews.llvm.org/D51976#1237060
>
> [2] http://lists.llvm.org/pipermail/llvm-dev/2019-November/137021.html
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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(+usual debug info folks)

I'm mostly staying out of discussions around optimized debug info/variable
locations - other folks have more state on this than I do. But some casual
thoughts from the peanut gallery...

On Thu, Feb 20, 2020 at 8:54 AM Tozer, Stephen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Currently, the debug intrinsic functions each have 3 arguments: an SSA
> value representing either the address or Value of a local variable, a
> DILocalVariable, and a complex expression. If the SSA value is an
> Instruction, and that Instruction is at some point deleted, we attempt to
> salvage the SSA value by recreating the instruction within the complex
> expression. If the instruction cannot be replicated by a complex
> expression, then the variable location is dropped causing a reduction in
> coverage. One of the key restrictions on this process at the moment is that
> each intrinsic can only reference a single SSA value; a numeric constant
> may be represented within the expression itself, allowing for binary
> operators with a constant operand to be salvaged:
>
>
>
> %c = add i32 %a, 4
>
> llvm.dbg.value(metadata i32 %c, DILocalVariable("x"),
DIExpression())
>
> ; Salvage...
>
> llvm.dbg.value(metadata i32 %a, DILocalVariable("x"),
> DIExpression(DW_OP_constu, 4, DW_OP_plus))
>
>
>
> This proposal is to allow multiple SSA value references within a debug
> intrinsic, allowing binary operators with non-constant operands to be
> salvaged. This is a long-awaited feature, with an open bugzilla[0] and
> support from members of the community[1][2]. To implement this change, each
> debug intrinsic will contain a list of SSA values instead of just one, and
> a new operator will be added: DW_OP_LLVM_register, which takes as its
> only argument the index of an SSA value within the intrinsic’s list, and
> pushes that SSA value onto the expression stack.
>
What would it look like without this extension? If we modeled it as if all
the register values were already on the stack (an extension of the current
way where the singular value is modeled as being already on the stack, if I
understand it correctly?)?

If it's decided that the best approach is to introduce something like
DW_OP_LLVM_register - might be worth migrating to that first (basically
adding DW_OP_LLVM_register(0) at the start of every DIExpression) and then
expanding it from unary to n-ary support.

> Two proposed syntaxes for the list of SSA values - though suitable
> alternatives may be worth considering - are to either replace the first
> argument of the intrinsic function with an MDNode containing the SSA values
> as operands, or to remove the first argument and make the intrinsic
> function variadic, passing the SSA value list as vargs:
>
>
>
> %c = add i32 %a, %b
>
> llvm.dbg.value(metadata i32 %c, DILocalVariable("x"),
DIExpression())
>
> ; Salvage...
>
> llvm.dbg.value(!{metadata i32 %a, metadata i32 %b},
DILocalVariable("x"),
> DIExpression(DW_OP_LLVM_register, 0, DW_OP_LLVM_register, 1, DW_OP_plus))
>
> ; Alternatively, the intrinsic function could be made variadic...
>
> llvm.dbg.value(DILocalVariable("x"),
DIExpression(DW_OP_LLVM_register, 0,
> DW_OP_LLVM_register, 1, DW_OP_plus), metadata i32 %a, metadata i32 %b)
>
>
>
> The new operator DW_OP_LLVM_register would exist in the IR and MIR, and
> further down the pipeline be replaced by the appropriate operator for the
> target debug output. For example, when producing DWARF this would be
> replaced by DW_OP_regval_type, which pushes the contents of a given
> register interpreted as a value of a given type onto the DWARF expression
> stack.
>
>
>
> This has the potential to allow salvaging in a much greater number of
> cases than is currently possible. There are also potential follow-up tasks,
> such as allowing the salvaging of conditional values, that would further
> improve debug variable availability. The following table gives, for several
> of the multi-source application projects in the test suite, the number of
> successful invocations of SalvageDebugInfo, and the number of failed
> salvages for each type of unsalvageable instruction:
>
>
>
>             Success             Variadic Binops     Variadic GEPs
> Cmp Insts           Select Insts        Load Insts          Phi
> Nodes           Alloca Insts        Call Insts
>
> ALAC        261                 29                  61
> 0                   0                   1
> 12                  0                   0
>
> Burg        50                  1                   9
> 0                   1                   95
>  6                   0                   0
>
> hbd         514                 16                  1
> 0                   3                   45
> 10                  0                   4
>
> Lua         270                 12                  54
> 0                   12                  46
> 32                  1                   0
>
> minisat     458                 10                  10
> 3                   1                   35
>            4                   0                   0
>
> sgefa       439                 1                   121
> 0                   20                  14
> 55                  0                   0
>
> SIBsim4     153                 15                  6
> 0                   3                   40
> 3                   0                   0
>
> siod        112                 2                   1
> 0                   0                   11
> 5                   2                   1
>
> SPASS       1241                70                  27
> 27                  27                  2114
> 156                 0                   7
>
> spiff       39                  0                   15
> 0                   0                   7
> 2                   1                   0
>
> sqlite3     2322                94                  167
> 6                   37                  1143
> 136                 4                   10
>
> treecc      127                 1                   0
> 0                   1                   350
> 37                  0                   1
>
> viterbi     7                   0                   1
> 0                   1                   1
> 0                   0                   0
>
>
>
>
>
> Of these categories, the first 3 will become salvageable after this work
> is implemented, and Select Insts will also be salvageable with some
> follow-up work to enable conditional branching in complex expressions. The
> remainder are not salvageable in general, although it's possible that
the
> specific passes that delete those instructions may be able to preserve the
> debug info as long as the code isn't totally dead.
>
>
>
> [0] https://bugs.llvm.org/show_bug.cgi?id=39141
>
> [1] https://reviews.llvm.org/D51976#1237060
>
> [2] http://lists.llvm.org/pipermail/llvm-dev/2019-November/137021.html
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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What would it look like without this extension? If we modeled it as if all the
register values were already on the stack (an extension of the current way where
the singular value is modeled as being already on the stack, if I understand it
correctly?)?

If it's decided that the best approach is to introduce something like
DW_OP_LLVM_register - might be worth migrating to that first (basically adding
DW_OP_LLVM_register(0) at the start of every DIExpression) and then expanding it
from unary to n-ary support
Putting the register values initially on the stack reduces the verbosity, though
it could complicate successive salvages of variadic DIExpressions - if any value
other than the last needs salvaging, then you have to use DWARF stack operations
to move it to the top of the stack. For example, if the elements are pushed in
order so that the last element is on the top of the stack:

%c = mul 3, %a
%d = add 5, %b
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_plus), %c, %d)
; Salvage %d
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_plus_constu, 5,
DW_OP_plus), %c, %b)
; Salvage %c needs to use DW_OP_swap
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_plus_constu, 5,
DW_OP_swap, DW_OP_constu, 3, DW_OP_mul, DW_OP_plus), %a, %b)

Or, written out as the stack state:
[%a, %b]               ; Initial state
[%a, %b + 5]           ; DW_OP_plus_constu, 5
[%b + 5, %a]           ; DW_OP_swap
[%b + 5, %a, 3]        ; DW_OP_constu, 3
[%b + 5, %a * 3]       ; DW_OP_mul
[(%b + 5) + (%a * 3)]  ; DW_OP_plus

The simplest and most general solution would be to use DW_OP_pick, which
duplicates the stack element at a given index to the top of the stack. This is
more or less the same as using DW_OP_LLVM_register - both of them take an index
corresponding to a register value - with a few differences: we get to maintain
the default concise dbg.value with a single argument and empty DIExpression, but
salvaging becomes more brittle. If we rely on elements being on the stack in
their declared order, then we need to guarantee that nothing modifies those
stack elements. The cleanest implementation of this would be for
SalvageDebugInfo to salvage expressions normally when the last register is
salvaged, and switch to using DW_OP_pick for every register whenever any other
register is salvaged:

%c = add %a, 5
%e = div %c, %d
; x = %b * ((%a + 5) / %d)
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_mul), %b, %e)
; Salvage %e; last operator so salvage normally
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_mul, DW_OP_div),
%b, %c, %d)
; Salvage %c; not last operator and expression isn't already using pick, so
add DW_OP_pick for registers
dbg.value(!DILocalVariable("x"), !DIExpression(DW_OP_pick, 0,
DW_OP_pick, 1, DW_OP_plus_constu, 5, DW_OP_pick, 2, DW_OP_div, DW_OP_mul), %b,
%a, %d)

The major advantage of putting all registers on the stack is that it reduces
verbosity and also doesn't require us to implement the new operator or
update existing DIExpressions to contain it, which is useful. Personally I lean
towards keeping things simple and consistent, and so using the pick/register
operator in every expression rather than only the ones that need it, but
there's a good case for either I think.

________________________________
From: David Blaikie <dblaikie at gmail.com>
Sent: 20 February 2020 20:21
To: Tozer, Stephen <stephen.tozer at sony.com>; Adrian Prantl <aprantl
at apple.com>; Jonas Devlieghere <jdevlieghere at apple.com>; Robinson,
Paul <paul.robinson at sony.com>; Eric Christopher <echristo at
gmail.com>
Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Allowing debug intrinsics to reference multiple
SSA Values

(+usual debug info folks)

I'm mostly staying out of discussions around optimized debug info/variable
locations - other folks have more state on this than I do. But some casual
thoughts from the peanut gallery...

On Thu, Feb 20, 2020 at 8:54 AM Tozer, Stephen via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Currently, the debug intrinsic functions each have 3 arguments: an SSA value
representing either the address or Value of a local variable, a DILocalVariable,
and a complex expression. If the SSA value is an Instruction, and that
Instruction is at some point deleted, we attempt to salvage the SSA value by
recreating the instruction within the complex expression. If the instruction
cannot be replicated by a complex expression, then the variable location is
dropped causing a reduction in coverage. One of the key restrictions on this
process at the moment is that each intrinsic can only reference a single SSA
value; a numeric constant may be represented within the expression itself,
allowing for binary operators with a constant operand to be salvaged:



%c = add i32 %a, 4

llvm.dbg.value(metadata i32 %c, DILocalVariable("x"), DIExpression())

; Salvage...

llvm.dbg.value(metadata i32 %a, DILocalVariable("x"),
DIExpression(DW_OP_constu, 4, DW_OP_plus))



This proposal is to allow multiple SSA value references within a debug
intrinsic, allowing binary operators with non-constant operands to be salvaged.
This is a long-awaited feature, with an open bugzilla[0] and support from
members of the community[1][2]. To implement this change, each debug intrinsic
will contain a list of SSA values instead of just one, and a new operator will
be added: DW_OP_LLVM_register, which takes as its only argument the index of an
SSA value within the intrinsic’s list, and pushes that SSA value onto the
expression stack.

What would it look like without this extension? If we modeled it as if all the
register values were already on the stack (an extension of the current way where
the singular value is modeled as being already on the stack, if I understand it
correctly?)?

If it's decided that the best approach is to introduce something like
DW_OP_LLVM_register - might be worth migrating to that first (basically adding
DW_OP_LLVM_register(0) at the start of every DIExpression) and then expanding it
from unary to n-ary support.


Two proposed syntaxes for the list of SSA values - though suitable alternatives
may be worth considering - are to either replace the first argument of the
intrinsic function with an MDNode containing the SSA values as operands, or to
remove the first argument and make the intrinsic function variadic, passing the
SSA value list as vargs:



%c = add i32 %a, %b

llvm.dbg.value(metadata i32 %c, DILocalVariable("x"), DIExpression())

; Salvage...

llvm.dbg.value(!{metadata i32 %a, metadata i32 %b},
DILocalVariable("x"), DIExpression(DW_OP_LLVM_register, 0,
DW_OP_LLVM_register, 1, DW_OP_plus))

; Alternatively, the intrinsic function could be made variadic...

llvm.dbg.value(DILocalVariable("x"), DIExpression(DW_OP_LLVM_register,
0, DW_OP_LLVM_register, 1, DW_OP_plus), metadata i32 %a, metadata i32 %b)



The new operator DW_OP_LLVM_register would exist in the IR and MIR, and further
down the pipeline be replaced by the appropriate operator for the target debug
output. For example, when producing DWARF this would be replaced by
DW_OP_regval_type, which pushes the contents of a given register interpreted as
a value of a given type onto the DWARF expression stack.



This has the potential to allow salvaging in a much greater number of cases than
is currently possible. There are also potential follow-up tasks, such as
allowing the salvaging of conditional values, that would further improve debug
variable availability. The following table gives, for several of the
multi-source application projects in the test suite, the number of successful
invocations of SalvageDebugInfo, and the number of failed salvages for each type
of unsalvageable instruction:



            Success             Variadic Binops     Variadic GEPs       Cmp
Insts           Select Insts        Load Insts          Phi Nodes          
Alloca Insts        Call Insts

ALAC        261                 29                  61                  0       
0                   1                   12                  0                  
0

Burg        50                  1                   9                   0       
1                   95                  6                   0                  
0

hbd         514                 16                  1                   0       
3                   45                  10                  0                  
4

Lua         270                 12                  54                  0       
12                  46                  32                  1                  
0

minisat     458                 10                  10                  3       
1                   35                  4                   0                  
0

sgefa       439                 1                   121                 0       
20                  14                  55                  0                  
0

SIBsim4     153                 15                  6                   0       
3                   40                  3                   0                  
0

siod        112                 2                   1                   0       
0                   11                  5                   2                  
1

SPASS       1241                70                  27                  27      
27                  2114                156                 0                  
7

spiff       39                  0                   15                  0       
0                   7                   2                   1                  
0

sqlite3     2322                94                  167                 6       
37                  1143                136                 4                  
10

treecc      127                 1                   0                   0       
1                   350                 37                  0                  
1

viterbi     7                   0                   1                   0       
1                   1                   0                   0                  
0





Of these categories, the first 3 will become salvageable after this work is
implemented, and Select Insts will also be salvageable with some follow-up work
to enable conditional branching in complex expressions. The remainder are not
salvageable in general, although it's possible that the specific passes that
delete those instructions may be able to preserve the debug info as long as the
code isn't totally dead.



[0] https://bugs.llvm.org/show_bug.cgi?id=39141

[1] https://reviews.llvm.org/D51976#1237060

[2] http://lists.llvm.org/pipermail/llvm-dev/2019-November/137021.html

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