[PATCH] D115247: [EarlyCSE] Retain poison flags, if program is UB if poison.

[Florian Hahn via Phabricator via llvm-commits]

fhahn updated this revision to Diff 393567.
fhahn marked an inline comment as done.
fhahn added a comment.

Check for FPMathOperator.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D115247/new/

https://reviews.llvm.org/D115247

Files:
  llvm/lib/Transforms/Scalar/EarlyCSE.cpp
  llvm/test/Transforms/EarlyCSE/flags.ll


Index: llvm/test/Transforms/EarlyCSE/flags.ll
===================================================================
--- llvm/test/Transforms/EarlyCSE/flags.ll
+++ llvm/test/Transforms/EarlyCSE/flags.ll
@@ -22,7 +22,7 @@
 
 define void @test_inbounds_program_ub_if_first_gep_poison(i8* %ptr, i64 %n) {
 ; CHECK-LABEL: @test_inbounds_program_ub_if_first_gep_poison(
-; CHECK-NEXT:    [[ADD_PTR_1:%.*]] = getelementptr i8, i8* [[PTR:%.*]], i64 [[N:%.*]]
+; CHECK-NEXT:    [[ADD_PTR_1:%.*]] = getelementptr inbounds i8, i8* [[PTR:%.*]], i64 [[N:%.*]]
 ; CHECK-NEXT:    call void @use.i8(i8* noundef [[ADD_PTR_1]])
 ; CHECK-NEXT:    call void @use.i8(i8* [[ADD_PTR_1]])
 ; CHECK-NEXT:    ret void
Index: llvm/lib/Transforms/Scalar/EarlyCSE.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -1366,8 +1366,17 @@
           LLVM_DEBUG(dbgs() << "Skipping due to debug counter\n");
           continue;
         }
-        if (auto *I = dyn_cast<Instruction>(V))
-          I->andIRFlags(&Inst);
+        if (auto *I = dyn_cast<Instruction>(V)) {
+          // If I being poison triggers UB, there is no need to drop those
+          // flags. Otherwise, only retain flags present on both I and Inst.
+          // TODO: Currently some fast-math flags are not treated as
+          // poison-generating even though they should. Until this is fixed,
+          // always retain flags present on both I and Inst for floating point
+          // instructions.
+          if (isa<FPMathOperator>(I) || !I->hasPoisonGeneratingFlags() ||
+              !programUndefinedIfPoison(I))
+            I->andIRFlags(&Inst);
+        }
         Inst.replaceAllUsesWith(V);
         salvageKnowledge(&Inst, &AC);
         removeMSSA(Inst);


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D115247.393567.patch
Type: text/x-patch
Size: 1828 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20211210/93ed3792/attachment-0001.bin>