[IndVarSimplify] Fix poison-safety when reusing instructions

IndVars may replace an instruction with one of its operands, if
they have the same SCEV expression. However, such a replacement
may be more poisonous.

First, check whether the operand being poison implies that the
instruction is also poison, in which case the replacement is
always safe. If this fails, check whether SCEV can determine that
reusing the instruction is safe, using the same check as
SCEVExpander.

Fixes llvm#79861.

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -16,6 +16,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
@@ -713,8 +714,11 @@ bool SimplifyIndvar::replaceFloatIVWithIntegerIV(Instruction *UseInst) {
 bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
                                            Instruction *IVOperand) {
   if (!SE->isSCEVable(UseInst->getType()) ||
-      (UseInst->getType() != IVOperand->getType()) ||
-      (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
+      UseInst->getType() != IVOperand->getType())
+    return false;
+
+  const SCEV *UseSCEV = SE->getSCEV(UseInst);
+  if (UseSCEV != SE->getSCEV(IVOperand))
     return false;
 
   // getSCEV(X) == getSCEV(Y) does not guarantee that X and Y are related in the
@@ -742,6 +746,16 @@ bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
   if (!LI->replacementPreservesLCSSAForm(UseInst, IVOperand))
     return false;
 
+  // Make sure the operand is not more poisonous than the instruction.
+  if (!impliesPoison(IVOperand, UseInst)) {
+    SmallVector<Instruction *> DropPoisonGeneratingInsts;
+    if (!SE->canReuseInstruction(UseSCEV, IVOperand, DropPoisonGeneratingInsts))
+      return false;
+
+    for (Instruction *I : DropPoisonGeneratingInsts)
+      I->dropPoisonGeneratingFlagsAndMetadata();
+  }
+
   LLVM_DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
 
   SE->forgetValue(UseInst);

llvm/test/Transforms/IndVarSimplify/pr55925.ll

@@ -18,9 +18,9 @@ define void @test(ptr %p) personality ptr undef {
 ; CHECK-NEXT:    [[RES:%.*]] = invoke i32 @foo(i32 returned [[TMP0]])
 ; CHECK-NEXT:            to label [[LOOP_LATCH]] unwind label [[EXIT:%.*]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[INDVARS_IV]] to i32
 ; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @foo(i32 [[TMP1]])
+; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    [[LP:%.*]] = landingpad { ptr, i32 }
@@ -64,8 +64,8 @@ define void @test_critedge(i1 %c, ptr %p) personality ptr undef {
 ; CHECK-NEXT:    br label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ [[TMP1]], [[LOOP_INVOKE]] ], [ 0, [[LOOP_OTHER]] ]
-; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @foo(i32 [[PHI]])
+; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    [[LP:%.*]] = landingpad { ptr, i32 }

llvm/test/Transforms/IndVarSimplify/pr79861.ll

@@ -12,7 +12,9 @@ define void @or_disjoint() {
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 2, [[ENTRY:%.*]] ], [ [[IV_DEC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[OR:%.*]] = or disjoint i64 [[IV]], 1
-; CHECK-NEXT:    call void @use(i64 [[OR]])
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 false, i64 [[OR]], i64 [[ADD]]
+; CHECK-NEXT:    call void @use(i64 [[SEL]])
 ; CHECK-NEXT:    [[IV_DEC]] = add nsw i64 [[IV]], -1
 ; CHECK-NEXT:    [[EXIT_COND:%.*]] = icmp eq i64 [[IV_DEC]], 0
 ; CHECK-NEXT:    br i1 [[EXIT_COND]], label [[EXIT:%.*]], label [[LOOP]]
@@ -44,7 +46,7 @@ define void @add_nowrap_flags(i64 %n) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_INC:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[ADD1:%.*]] = add nuw nsw i64 [[IV]], 123
+; CHECK-NEXT:    [[ADD1:%.*]] = add i64 [[IV]], 123
 ; CHECK-NEXT:    call void @use(i64 [[ADD1]])
 ; CHECK-NEXT:    [[IV_INC]] = add i64 [[IV]], 1
 ; CHECK-NEXT:    [[EXIT_COND:%.*]] = icmp eq i64 [[IV_INC]], [[N]]