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

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: nikic, reames, nlopes, spatel.
Herald added a subscriber: hiraditya.
fhahn requested review of this revision.
Herald added a project: LLVM.

Poison-generating flags can be retained during CSE on the earlier
instruction , *if* the earlier instruction being poison causes UB.

Note that the code as written assumes that an instruction cannot have
poison-generating flags and fast-math flags, but I am not sure this is
always true. Should we add a version of `andIRFlags` that selectively
allows AND'ing fast-math flags only?

https://alive2.llvm.org/ce/z/4K3D7P


Repository:
  rG LLVM Github Monorepo

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 +1366,10 @@
           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->hasPoisonGeneratingFlags() || !programUndefinedIfPoison(I))
+            I->andIRFlags(&Inst);
+        }
         Inst.replaceAllUsesWith(V);
         salvageKnowledge(&Inst, &AC);
         removeMSSA(Inst);


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