[llvm] c2eccc6 - [GuardWidening] Remove nuw/nsw flags for hoisted instructions

[Serguei Katkov via llvm-commits]

Author: Serguei Katkov
Date: 2022-05-26T13:20:55+07:00
New Revision: c2eccc67ce07e9cb374eb0ecdb3038fcb8be08cd

URL: https://github.com/llvm/llvm-project/commit/c2eccc67ce07e9cb374eb0ecdb3038fcb8be08cd
DIFF: https://github.com/llvm/llvm-project/commit/c2eccc67ce07e9cb374eb0ecdb3038fcb8be08cd.diff

LOG: [GuardWidening] Remove nuw/nsw flags for hoisted instructions

When we hoist instructions over guard we must clear flags due to these flags
might be implied using this guard, so they make sense only after the guard.

As an example of the bug due to current behavior.
L is known to be in range say [0, 100)
c1 = x u< L
guard (c1)
x1 = add x, 1
c2 = x1 u< L
guard(c2)

basing on guard(c1) we can say that x1 = add nuw nsw x, 1
after guard widening we get
c1 = x u< L
x1 = add nuw nsw x, 1
c2 = x1 u< L
c = and c1, c2
guard(c)

now, basing on fact that x + 1 < L and x >= 0 due to x + 1 is nuw
we can prove that x + 1 u< L implies that x u< L, so we can just remove c1
x1 = add nuw nsw x, 1
c2 = x1 u< L
guard(c2)

But that is not correct due to we will pass x == -1 value.

Reviewed By: mkazantsev
Subscribers: llvm-commits, nikic
Differential Revision: https://reviews.llvm.org/D126354

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/GuardWidening.cpp
    llvm/test/Transforms/GuardWidening/range-check-merging.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/GuardWidening.cpp b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
index 40c99e17ffea2..5032f3106d50c 100644
--- a/llvm/lib/Transforms/Scalar/GuardWidening.cpp
+++ b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
@@ -495,6 +495,9 @@ void GuardWideningImpl::makeAvailableAt(Value *V, Instruction *Loc) const {
     makeAvailableAt(Op, Loc);
 
   Inst->moveBefore(Loc);
+  // If we moved instruction before guard we must clean nuw, nsw flags.
+  Inst->setHasNoUnsignedWrap(false);
+  Inst->setHasNoSignedWrap(false);
 }
 
 bool GuardWideningImpl::widenCondCommon(Value *Cond0, Value *Cond1,

diff  --git a/llvm/test/Transforms/GuardWidening/range-check-merging.ll b/llvm/test/Transforms/GuardWidening/range-check-merging.ll
index b4ab9b32ff48b..b2e229fd72110 100644
--- a/llvm/test/Transforms/GuardWidening/range-check-merging.ll
+++ b/llvm/test/Transforms/GuardWidening/range-check-merging.ll
@@ -340,7 +340,7 @@ define void @f_8(i32 %x, i32* %length_buf) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[LENGTH:%.*]] = load i32, i32* [[LENGTH_BUF:%.*]], align 4, !range [[RNG0]]
 ; CHECK-NEXT:    [[CHK0:%.*]] = icmp ult i32 [[X:%.*]], [[LENGTH]]
-; CHECK-NEXT:    [[X_INC1:%.*]] = add nuw nsw i32 [[X]], 1
+; CHECK-NEXT:    [[X_INC1:%.*]] = add i32 [[X]], 1
 ; CHECK-NEXT:    [[CHK1:%.*]] = icmp ult i32 [[X_INC1]], [[LENGTH]]
 ; CHECK-NEXT:    [[WIDE_CHK:%.*]] = and i1 [[CHK0]], [[CHK1]]
 ; CHECK-NEXT:    call void (i1, ...) @llvm.experimental.guard(i1 [[WIDE_CHK]]) [ "deopt"() ]