llvm dev - Feb 2019 - [RFC] Potential extension to asm statement functionality

The team I am working with is using asm statements containing label definitions
as a way of instrumentation so that when an application is loaded into their
debug and test framework, the labels will cause breakpoints to be set at
strategic points where they can query the state of the processor that the
application is running on.

~ Todd

From: Eli Friedman [mailto:efriedma at quicinc.com]
Sent: Tuesday, February 12, 2019 1:36 PM
To: Snider, Todd; llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

Can you go into a bit more detail about why someone would want to do this, as
opposed to just writing a file scope inline asm, or a separate file in assembly?
I can't think of any practical use for the fact that the label is
"inside" the function body.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 10:18 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Potential extension to asm statement
functionality


Suppose a programmer wants to inject their own global label definition into the
body of a function with some guarantee that it will not be removed by the
compiler.

One way to do this is to define a global label with an asm statement knowing
that the asm statement will not be invoked until after the compiler's
optimization passes have run, but the following case demonstrates that a label
defined with an asm statement is still susceptible to being duplicated:

#include <stdint.h>
uint32_t f(uint32_t x);
uint32_t g(uint32_t x);

uint32_t f(uint32_t x) {
    uint32_t returnValue = g(x);

    if (returnValue > 0U) {
        returnValue = 0x40000000;
    }
    else {
        returnValue = 0x80000000;
    }
    __asm __volatile__ ("\t.global my_hook_fcn\nmy_hook_fcn:\n");
    return returnValue;
}

uint32_t g(uint32_t x) {
    return x >> 1U;
}

If the above definition of f() is compiled with optimization at level 1 or
higher, the TailDuplication optimization pass will duplicate and move the asm
statement and return up into the if block and the else block before the asm
statement is invoked. When the now duplicate asm statements are later invoked,
the compiler will detect a symbol redefinition error.

To address this situation, the asm statement functionality could be extended to
comprehend whether it contains a label definition, and if it does, to disallow
duplication of the asm statement.

There are a couple of different approaches that could be taken to implement
this:

  1.  Parse the content of the assembly string argument to the asm statement in
the compiler front-end (during EmitAsmStmt(), for example) to determine if it
contains a label definition, and if it does set the isNotDuplicable flag on the
INLINEASM record that is created to represent the asm statement in the IR. To
date, there is no precedence for processing the content of the assembly string
argument until the asm statement is invoked before the integrated  assembler
starts processing the generated machine code.
  2.  Add a label constraint to the input and output operand syntax for asm
statements. i.e.
__asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));
The "lbl" constraint would tell the compiler to mark the asm statement
as isNotDuplicable when an INLINEASM record is created to represent it

There are alternatives to using an asm statement for this purpose, but I
wondered if such an extension would be useful/valuable to the wider community.

Thoughts? Opinions?

Todd Snider

Compiler Group
Texas Instruments Incorporated
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This tactic has been tried before, I'm pretty sure.  It's an attempt to
do simple instrumentation without having to hack the compiler itself (you do
some manual coding or preprocessing, and poof you get useful info in your object
file).  But our compiler is too clever.

Some sort of "No Touchee!" constraint would help this use-case a lot.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 3:06 PM
To: Eli Friedman; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] Potential extension to asm statement functionality


The team I am working with is using asm statements containing label definitions
as a way of instrumentation so that when an application is loaded into their
debug and test framework, the labels will cause breakpoints to be set at
strategic points where they can query the state of the processor that the
application is running on.

~ Todd

From: Eli Friedman [mailto:efriedma at quicinc.com]
Sent: Tuesday, February 12, 2019 1:36 PM
To: Snider, Todd; llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

Can you go into a bit more detail about why someone would want to do this, as
opposed to just writing a file scope inline asm, or a separate file in assembly?
I can't think of any practical use for the fact that the label is
"inside" the function body.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 10:18 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Potential extension to asm statement
functionality


Suppose a programmer wants to inject their own global label definition into the
body of a function with some guarantee that it will not be removed by the
compiler.

One way to do this is to define a global label with an asm statement knowing
that the asm statement will not be invoked until after the compiler's
optimization passes have run, but the following case demonstrates that a label
defined with an asm statement is still susceptible to being duplicated:

#include <stdint.h>
uint32_t f(uint32_t x);
uint32_t g(uint32_t x);

uint32_t f(uint32_t x) {
    uint32_t returnValue = g(x);

    if (returnValue > 0U) {
        returnValue = 0x40000000;
    }
    else {
        returnValue = 0x80000000;
    }
    __asm __volatile__ ("\t.global my_hook_fcn\nmy_hook_fcn:\n");
    return returnValue;
}

uint32_t g(uint32_t x) {
    return x >> 1U;
}

If the above definition of f() is compiled with optimization at level 1 or
higher, the TailDuplication optimization pass will duplicate and move the asm
statement and return up into the if block and the else block before the asm
statement is invoked. When the now duplicate asm statements are later invoked,
the compiler will detect a symbol redefinition error.

To address this situation, the asm statement functionality could be extended to
comprehend whether it contains a label definition, and if it does, to disallow
duplication of the asm statement.

There are a couple of different approaches that could be taken to implement
this:

  1.  Parse the content of the assembly string argument to the asm statement in
the compiler front-end (during EmitAsmStmt(), for example) to determine if it
contains a label definition, and if it does set the isNotDuplicable flag on the
INLINEASM record that is created to represent the asm statement in the IR. To
date, there is no precedence for processing the content of the assembly string
argument until the asm statement is invoked before the integrated  assembler
starts processing the generated machine code.
  2.  Add a label constraint to the input and output operand syntax for asm
statements. i.e.
__asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));
The "lbl" constraint would tell the compiler to mark the asm statement
as isNotDuplicable when an INLINEASM record is created to represent it

There are alternatives to using an asm statement for this purpose, but I
wondered if such an extension would be useful/valuable to the wider community.

Thoughts? Opinions?

Todd Snider

Compiler Group
Texas Instruments Incorporated
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The proposed "lbl" constraint below:
   __asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));

is not quite a "No Touchie!" constraint, but it does allow the user to
set the isNotDuplicable flag on the INLINEASM that comes out of the asm
statement in order to circumvent optimizations like Tail Duplication.  But
setting the isNotDuplicable flag is not really enough. If the function that
contains the "lbl" constrained asm statement is inlined into a
function in the same compilation unit, the compiler will error out with a symbol
redefinition error.

We may need something on the level of a "No Touchie!" constraint.

Perhaps a new builtin function that resolves to a global label definition is a
better alternative for this use case?

~ Todd

From: paul.robinson at sony.com [mailto:paul.robinson at sony.com]
Sent: Tuesday, February 12, 2019 2:32 PM
To: Snider, Todd; efriedma at quicinc.com
Cc: llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

This tactic has been tried before, I'm pretty sure.  It's an attempt to
do simple instrumentation without having to hack the compiler itself (you do
some manual coding or preprocessing, and poof you get useful info in your object
file).  But our compiler is too clever.

Some sort of "No Touchee!" constraint would help this use-case a lot.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 3:06 PM
To: Eli Friedman; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] Potential extension to asm statement functionality


The team I am working with is using asm statements containing label definitions
as a way of instrumentation so that when an application is loaded into their
debug and test framework, the labels will cause breakpoints to be set at
strategic points where they can query the state of the processor that the
application is running on.

~ Todd

From: Eli Friedman [mailto:efriedma at quicinc.com]
Sent: Tuesday, February 12, 2019 1:36 PM
To: Snider, Todd; llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

Can you go into a bit more detail about why someone would want to do this, as
opposed to just writing a file scope inline asm, or a separate file in assembly?
I can't think of any practical use for the fact that the label is
"inside" the function body.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 10:18 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Potential extension to asm statement
functionality


Suppose a programmer wants to inject their own global label definition into the
body of a function with some guarantee that it will not be removed by the
compiler.

One way to do this is to define a global label with an asm statement knowing
that the asm statement will not be invoked until after the compiler's
optimization passes have run, but the following case demonstrates that a label
defined with an asm statement is still susceptible to being duplicated:

#include <stdint.h>
uint32_t f(uint32_t x);
uint32_t g(uint32_t x);

uint32_t f(uint32_t x) {
    uint32_t returnValue = g(x);

    if (returnValue > 0U) {
        returnValue = 0x40000000;
    }
    else {
        returnValue = 0x80000000;
    }
    __asm __volatile__ ("\t.global my_hook_fcn\nmy_hook_fcn:\n");
    return returnValue;
}

uint32_t g(uint32_t x) {
    return x >> 1U;
}

If the above definition of f() is compiled with optimization at level 1 or
higher, the TailDuplication optimization pass will duplicate and move the asm
statement and return up into the if block and the else block before the asm
statement is invoked. When the now duplicate asm statements are later invoked,
the compiler will detect a symbol redefinition error.

To address this situation, the asm statement functionality could be extended to
comprehend whether it contains a label definition, and if it does, to disallow
duplication of the asm statement.

There are a couple of different approaches that could be taken to implement
this:

  1.  Parse the content of the assembly string argument to the asm statement in
the compiler front-end (during EmitAsmStmt(), for example) to determine if it
contains a label definition, and if it does set the isNotDuplicable flag on the
INLINEASM record that is created to represent the asm statement in the IR. To
date, there is no precedence for processing the content of the assembly string
argument until the asm statement is invoked before the integrated  assembler
starts processing the generated machine code.
  2.  Add a label constraint to the input and output operand syntax for asm
statements. i.e.
__asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));
The "lbl" constraint would tell the compiler to mark the asm statement
as isNotDuplicable when an INLINEASM record is created to represent it

There are alternatives to using an asm statement for this purpose, but I
wondered if such an extension would be useful/valuable to the wider community.

Thoughts? Opinions?

Todd Snider

Compiler Group
Texas Instruments Incorporated
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Hi Paul,

Regarding the "No Touchie!" constraint idea for asm statements: would
this be a new qualifier (like volatile) that could be applied to the asm
statement?
Since the constraint is not necessarily associated with an input or output
operand, it seems that introducing the constraint via the qualifier field might
work.

All,

The volatile qualifier on an asm statement already indicates that the statement
should be assumed to have side effects.
Could we extend the functionality of volatile to also mean the asm statement is
not duplicable?

~ Todd

From: Snider, Todd
Sent: Wednesday, February 13, 2019 11:40 AM
To: 'paul.robinson at sony.com'; efriedma at quicinc.com
Cc: llvm-dev at lists.llvm.org
Subject: RE: [llvm-dev] [RFC] Potential extension to asm statement functionality


The proposed "lbl" constraint below:
   __asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));

is not quite a "No Touchie!" constraint, but it does allow the user to
set the isNotDuplicable flag on the INLINEASM that comes out of the asm
statement in order to circumvent optimizations like Tail Duplication.  But
setting the isNotDuplicable flag is not really enough. If the function that
contains the "lbl" constrained asm statement is inlined into a
function in the same compilation unit, the compiler will error out with a symbol
redefinition error.

We may need something on the level of a "No Touchie!" constraint.

Perhaps a new builtin function that resolves to a global label definition is a
better alternative for this use case?

~ Todd

From: paul.robinson at sony.com [mailto:paul.robinson at sony.com]
Sent: Tuesday, February 12, 2019 2:32 PM
To: Snider, Todd; efriedma at quicinc.com
Cc: llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

This tactic has been tried before, I'm pretty sure.  It's an attempt to
do simple instrumentation without having to hack the compiler itself (you do
some manual coding or preprocessing, and poof you get useful info in your object
file).  But our compiler is too clever.

Some sort of "No Touchee!" constraint would help this use-case a lot.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 3:06 PM
To: Eli Friedman; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] Potential extension to asm statement functionality


The team I am working with is using asm statements containing label definitions
as a way of instrumentation so that when an application is loaded into their
debug and test framework, the labels will cause breakpoints to be set at
strategic points where they can query the state of the processor that the
application is running on.

~ Todd

From: Eli Friedman [mailto:efriedma at quicinc.com]
Sent: Tuesday, February 12, 2019 1:36 PM
To: Snider, Todd; llvm-dev at lists.llvm.org
Subject: [EXTERNAL] RE: [llvm-dev] [RFC] Potential extension to asm statement
functionality

Can you go into a bit more detail about why someone would want to do this, as
opposed to just writing a file scope inline asm, or a separate file in assembly?
I can't think of any practical use for the fact that the label is
"inside" the function body.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Snider,
Todd via llvm-dev
Sent: Tuesday, February 12, 2019 10:18 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Potential extension to asm statement
functionality


Suppose a programmer wants to inject their own global label definition into the
body of a function with some guarantee that it will not be removed by the
compiler.

One way to do this is to define a global label with an asm statement knowing
that the asm statement will not be invoked until after the compiler's
optimization passes have run, but the following case demonstrates that a label
defined with an asm statement is still susceptible to being duplicated:

#include <stdint.h>
uint32_t f(uint32_t x);
uint32_t g(uint32_t x);

uint32_t f(uint32_t x) {
    uint32_t returnValue = g(x);

    if (returnValue > 0U) {
        returnValue = 0x40000000;
    }
    else {
        returnValue = 0x80000000;
    }
    __asm __volatile__ ("\t.global my_hook_fcn\nmy_hook_fcn:\n");
    return returnValue;
}

uint32_t g(uint32_t x) {
    return x >> 1U;
}

If the above definition of f() is compiled with optimization at level 1 or
higher, the TailDuplication optimization pass will duplicate and move the asm
statement and return up into the if block and the else block before the asm
statement is invoked. When the now duplicate asm statements are later invoked,
the compiler will detect a symbol redefinition error.

To address this situation, the asm statement functionality could be extended to
comprehend whether it contains a label definition, and if it does, to disallow
duplication of the asm statement.

There are a couple of different approaches that could be taken to implement
this:

  1.  Parse the content of the assembly string argument to the asm statement in
the compiler front-end (during EmitAsmStmt(), for example) to determine if it
contains a label definition, and if it does set the isNotDuplicable flag on the
INLINEASM record that is created to represent the asm statement in the IR. To
date, there is no precedence for processing the content of the assembly string
argument until the asm statement is invoked before the integrated  assembler
starts processing the generated machine code.
  2.  Add a label constraint to the input and output operand syntax for asm
statements. i.e.
__asm __volatile__ ("\t.global\t%0\n%0:\n" : "lbl"
(my_hook_fcn));
The "lbl" constraint would tell the compiler to mark the asm statement
as isNotDuplicable when an INLINEASM record is created to represent it

There are alternatives to using an asm statement for this purpose, but I
wondered if such an extension would be useful/valuable to the wider community.

Thoughts? Opinions?

Todd Snider

Compiler Group
Texas Instruments Incorporated
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