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Teach LoopUnswitch not to perform non-trivial unswitching on loops containing convergent operations.

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Doing so could cause the post-unswitching convergent ops to be
control-dependent on the unswitch condition where they were not before.
This check could be refined to allow unswitching where the convergent
operation was already control-dependent on the unswitch condition.

llvm-svn: 249874
This commit is contained in:
Owen Anderson 2015-10-09 18:40:20 +00:00
parent 57bd7dc824
commit f8e3a19510
2 changed files with 53 additions and 0 deletions
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14
lib/Transforms/Scalar/LoopUnswitch.cpp
View File

@ -500,6 +500,20 @@ bool LoopUnswitch::processCurrentLoop() {
return true;
}
// Do not unswitch loops containing convergent operations, as we might be
// making them control dependent on the unswitch value when they were not
// before.
// FIXME: This could be refined to only bail if the convergent operation is
// not already control-dependent on the unswitch value.
for (const auto BB : currentLoop->blocks()) {
for (const auto &I : *BB) {
const auto CI = dyn_cast<CallInst>(&I);
if (!CI) continue;
if (CI->isConvergent())
return false;
}
}
// Do not do non-trivial unswitch while optimizing for size.
// FIXME: Use Function::optForSize().
if (OptimizeForSize ||

39
test/Transforms/LoopUnswitch/basictest.ll
View File

@ -64,5 +64,44 @@ loop_exit:
; CHECK: }
}
; This simple test would normally unswitch, but should be inhibited by the presence of
; the convergent call that is not control-dependent on the unswitch condition.
; CHECK-LABEL: @test3(
define i32 @test3(i32* %var) {
%mem = alloca i32
store i32 2, i32* %mem
%c = load i32, i32* %mem
br label %loop_begin
loop_begin:
%var_val = load i32, i32* %var
; CHECK: call void @conv()
; CHECK-NOT: call void @conv()
call void @conv() convergent
switch i32 %c, label %default [
i32 1, label %inc
i32 2, label %dec
]
inc:
call void @incf() noreturn nounwind
br label %loop_begin
dec:
call void @decf() noreturn nounwind
br label %loop_begin
default:
br label %loop_exit
loop_exit:
ret i32 0
; CHECK: }
}
declare void @incf() noreturn
declare void @decf() noreturn
declare void @conv() convergent