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[ValueTracking] Don't delete assumes of side-effectful instructions

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ValueTracking has to strike a balance when attempting to propagate information
backwards from assumes, because if the information is trivially propagated
backwards, it can appear to LLVM that the assumption is known to be true, and
therefore can be removed.

This is sound (because an assumption has no semantic effect except for causing
UB), but prevents the assume from allowing further optimizations.

The isEphemeralValueOf check exists to try and prevent this issue by not
removing the source of an assumption. This tries to make it a little bit more
general to handle the case of side-effectful instructions, such as in

  %0 = call i1 @get_val()
  %1 = xor i1 %0, true
  call void @llvm.assume(i1 %1)

Patch by Ariel Ben-Yehuda, thanks!

Differential Revision: https://reviews.llvm.org/D36590

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310859 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2017-08-14 17:11:43 +00:00
parent 1d2cebd945
commit 7d99ae532b
2 changed files with 19 additions and 8 deletions
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14
lib/Analysis/ValueTracking.cpp
View File

@ -384,13 +384,13 @@ static bool isEphemeralValueOf(const Instruction *I, const Value *E) {
if (V == E)
return true;
EphValues.insert(V);
if (const User *U = dyn_cast<User>(V))
for (User::const_op_iterator J = U->op_begin(), JE = U->op_end();
J != JE; ++J) {
if (isSafeToSpeculativelyExecute(*J))
WorkSet.push_back(*J);
}
if (V == I || isSafeToSpeculativelyExecute(V)) {
EphValues.insert(V);
if (const User *U = dyn_cast<User>(V))
for (User::const_op_iterator J = U->op_begin(), JE = U->op_end();
J != JE; ++J)
WorkSet.push_back(*J);
}
}
}

13
test/Analysis/ValueTracking/assume.ll
View File

@ -18,5 +18,16 @@ define i32 @assume_add(i32 %a, i32 %b) {
ret i32 %t3
}
declare void @llvm.assume(i1)
define void @assume_not() {
; CHECK-LABEL: @assume_not(
entry-block:
%0 = call i1 @get_val()
; CHECK: call void @llvm.assume
%1 = xor i1 %0, true
call void @llvm.assume(i1 %1)
ret void
}
declare i1 @get_val()
declare void @llvm.assume(i1)