[llvm] [instcombine] Drop zext nneg flag when simplify operand (PR #71088)

[Philip Reames via llvm-commits]

https://github.com/preames updated https://github.com/llvm/llvm-project/pull/71088

>From 26f6ec3e16ac4e2884e5ec56af13c8ab68998163 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Thu, 2 Nov 2023 11:19:19 -0700
Subject: [PATCH 1/2] [instcombine] Drop zext nneg flag when simplify operand

This fixes a miscompile introduced in the recent #67982, and likely exposed
in changes since to infer and leverage the same.  No active bug reports as
of yet.

This was noticed in https://github.com/llvm/llvm-project/pull/70858#discussion_r1378203532.
---
 .../Transforms/InstCombine/InstCombineSimplifyDemanded.cpp  | 6 +++++-
 llvm/test/Transforms/InstCombine/zext.ll                    | 2 +-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index fa6fe9d30abd1b8..30c1565ab44f7a5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -422,8 +422,12 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
 
     APInt InputDemandedMask = DemandedMask.zextOrTrunc(SrcBitWidth);
     KnownBits InputKnown(SrcBitWidth);
-    if (SimplifyDemandedBits(I, 0, InputDemandedMask, InputKnown, Depth + 1))
+    if (SimplifyDemandedBits(I, 0, InputDemandedMask, InputKnown, Depth + 1)) {
+      // For zext nneg, we may have dropped the instruction which made the
+      // input non-negative.
+      I->dropPoisonGeneratingFlags();
       return I;
+    }
     assert(InputKnown.getBitWidth() == SrcBitWidth && "Src width changed?");
     Known = InputKnown.zextOrTrunc(BitWidth);
     assert(!Known.hasConflict() && "Bits known to be one AND zero?");
diff --git a/llvm/test/Transforms/InstCombine/zext.ll b/llvm/test/Transforms/InstCombine/zext.ll
index 9dd0d929530b333..4034bc3c531b462 100644
--- a/llvm/test/Transforms/InstCombine/zext.ll
+++ b/llvm/test/Transforms/InstCombine/zext.ll
@@ -783,7 +783,7 @@ define i64 @evaluate_zexted_const_expr(i1 %c) {
 ; but the flag on the zext doesn't.
 define i16 @zext_nneg_flag_drop(i8 %x, i16 %y) {
 ; CHECK-LABEL: @zext_nneg_flag_drop(
-; CHECK-NEXT:    [[EXT:%.*]] = zext nneg i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[EXT:%.*]] = zext i8 [[X:%.*]] to i16
 ; CHECK-NEXT:    [[OR1:%.*]] = or i16 [[EXT]], [[Y:%.*]]
 ; CHECK-NEXT:    [[OR2:%.*]] = or i16 [[OR1]], 128
 ; CHECK-NEXT:    ret i16 [[OR2]]

>From dc64d3aeb6ba4f6a3fadab6da49bc5213415ae43 Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail at philipreames.com>
Date: Thu, 2 Nov 2023 11:43:23 -0700
Subject: [PATCH 2/2] Update zext.ll (drop fixme)

---
 llvm/test/Transforms/InstCombine/zext.ll | 2 --
 1 file changed, 2 deletions(-)

diff --git a/llvm/test/Transforms/InstCombine/zext.ll b/llvm/test/Transforms/InstCombine/zext.ll
index 4034bc3c531b462..5a00b7575d70fb4 100644
--- a/llvm/test/Transforms/InstCombine/zext.ll
+++ b/llvm/test/Transforms/InstCombine/zext.ll
@@ -779,8 +779,6 @@ define i64 @evaluate_zexted_const_expr(i1 %c) {
   ret i64 %ext
 }
 
-; FIXME: This is currently miscompiling as the and gets dropped,
-; but the flag on the zext doesn't.
 define i16 @zext_nneg_flag_drop(i8 %x, i16 %y) {
 ; CHECK-LABEL: @zext_nneg_flag_drop(
 ; CHECK-NEXT:    [[EXT:%.*]] = zext i8 [[X:%.*]] to i16