llvm dev - Dec 2015 - LICM doesn't work for IntrReadMem intrinsic function

Hi, All,

Suppose I define one memory read only intrinsic function "foo" in
Intrinsics.td like this

def int_foo : Intrinsic<[llvm_i32_ty], [llvm_i32_ty, llvm_i32_ty],
[IntrReadMem]>;

Suppose I have the following IR, which means the intrinsic function
"foo"
is called 10 times in a loop. Since the parameters of function "foo"
are
invariant and the function "foo" is defined as
"IntrReadMem". I assume the
'-licm' can move the function call 'foo' out of the loop body.
However,
after I use "opt test.ll -basicaa -licm -S", the function call
'foo' is
still inside the loop body. Do I misunderstand something here? Any
suggestions are appreciated. Thanks a lot.

; Function Attrs: uwtable
define i32 @_Z5func1i(i32 %a) #0 {
  br label %1

; <label>:1                                       ; preds = %3, %0
  %ret.0 = phi i32 [ 0, %0 ], [ %4, %3 ]
  %i.0 = phi i32 [ 0, %0 ], [ %5, %3 ]
  %2 = icmp slt i32 %i.0, 10
  br i1 %2, label %3, label %6

; <label>:3                                       ; preds = %1
  %4 = call i32 @llvm.foo(i32 %a, i32 3)
  %5 = add nsw i32 %i.0, 1
  br label %1

; <label>:6                                       ; preds = %1
  ret i32 %ret.0
}

; Function Attrs: nounwind readonly
declare i32 @llvm.foo(i32, i32) #1

Regards,

Xiangyang
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> On Dec 1, 2015, at 9:30 AM, Xiangyang Guo via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi, All,
> 
> Suppose I define one memory read only intrinsic function "foo" in
Intrinsics.td like this
> 
> def int_foo : Intrinsic<[llvm_i32_ty], [llvm_i32_ty, llvm_i32_ty],
[IntrReadMem]>;
> 
> Suppose I have the following IR, which means the intrinsic function
"foo" is called 10 times in a loop. Since the parameters of function
"foo" are invariant and the function "foo" is defined as
"IntrReadMem". I assume the '-licm' can move the function call
'foo' out of the loop body. However, after I use "opt test.ll
-basicaa -licm -S", the function call 'foo' is still inside the
loop body. Do I misunderstand something here? Any suggestions are appreciated.
Thanks a lot.
Does it work if you call -loop-rotate first? Loop unrolling requires loops to be
in canonicalized rotated form first, so I wouldn’t be surprised if LICM does as
well.

—escha

Thanks for your reply, escha,

Yes, -loop-rorate makes it work.

Regards,

Xiangyang

2015-12-01 12:48 GMT-05:00 <escha at apple.com>:
>
> > On Dec 1, 2015, at 9:30 AM, Xiangyang Guo via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi, All,
> >
> > Suppose I define one memory read only intrinsic function
"foo" in
> Intrinsics.td like this
> >
> > def int_foo : Intrinsic<[llvm_i32_ty], [llvm_i32_ty, llvm_i32_ty],
> [IntrReadMem]>;
> >
> > Suppose I have the following IR, which means the intrinsic function
> "foo" is called 10 times in a loop. Since the parameters of
function "foo"
> are invariant and the function "foo" is defined as
"IntrReadMem". I assume
> the '-licm' can move the function call 'foo' out of the
loop body. However,
> after I use "opt test.ll -basicaa -licm -S", the function call
'foo' is
> still inside the loop body. Do I misunderstand something here? Any
> suggestions are appreciated. Thanks a lot.
>
> Does it work if you call -loop-rotate first? Loop unrolling requires loops
> to be in canonicalized rotated form first, so I wouldn’t be surprised if
> LICM does as well.
>
> —escha
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