[SCEVExp] Keep NUW/NSW if both original inc and isomporphic inc agree. (#79512)

We are replacing with a wider increment. If both OrigInc and
IsomorphicInc are NUW/NSW, then we can preserve them on the wider
increment; the narrower IsomorphicInc would wrap before the wider
OrigInc, so the replacement won't make IsomorphicInc's uses more
poisonous.

PR: #79512

Author: fhahn

llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp

@@ -1604,11 +1604,36 @@ void SCEVExpander::replaceCongruentIVInc(
   const SCEV *TruncExpr =
       SE.getTruncateOrNoop(SE.getSCEV(OrigInc), IsomorphicInc->getType());
   if (OrigInc == IsomorphicInc || TruncExpr != SE.getSCEV(IsomorphicInc) ||
-      !SE.LI.replacementPreservesLCSSAForm(IsomorphicInc, OrigInc) ||
-      !hoistIVInc(OrigInc, IsomorphicInc,
+      !SE.LI.replacementPreservesLCSSAForm(IsomorphicInc, OrigInc))
+    return;
+
+  bool BothHaveNUW = false;
+  bool BothHaveNSW = false;
+  auto *OBOIncV = dyn_cast<OverflowingBinaryOperator>(OrigInc);
+  auto *OBOIsomorphic = dyn_cast<OverflowingBinaryOperator>(IsomorphicInc);
+  if (OBOIncV && OBOIsomorphic) {
+    BothHaveNUW =
+        OBOIncV->hasNoUnsignedWrap() && OBOIsomorphic->hasNoUnsignedWrap();
+    BothHaveNSW =
+        OBOIncV->hasNoSignedWrap() && OBOIsomorphic->hasNoSignedWrap();
+  }
+
+  if (!hoistIVInc(OrigInc, IsomorphicInc,
                   /*RecomputePoisonFlags*/ true))
     return;
 
+  // We are replacing with a wider increment. If both OrigInc and IsomorphicInc
+  // are NUW/NSW, then we can preserve them on the wider increment; the narrower
+  // IsomorphicInc would wrap before the wider OrigInc, so the replacement won't
+  // make IsomorphicInc's uses more poisonous.
+  assert(OrigInc->getType()->getScalarSizeInBits() >=
+             IsomorphicInc->getType()->getScalarSizeInBits() &&
+         "Should only replace an increment with a wider one.");
+  if (BothHaveNUW || BothHaveNSW) {
+    OrigInc->setHasNoUnsignedWrap(OBOIncV->hasNoUnsignedWrap() || BothHaveNUW);
+    OrigInc->setHasNoSignedWrap(OBOIncV->hasNoSignedWrap() || BothHaveNSW);
+  }
+
   SCEV_DEBUG_WITH_TYPE(DebugType,
                        dbgs() << "INDVARS: Eliminated congruent iv.inc: "
                               << *IsomorphicInc << '\n');

llvm/test/Transforms/IndVarSimplify/iv-poison.ll

@@ -64,7 +64,7 @@ define i2 @iv_hoist_both_adds_nsw(i2 %arg) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i2 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i2 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw nsw i2 [[IV_0]], 1
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i2 1, [[ARG:%.*]]
 ; CHECK-NEXT:    br i1 [[DOTNOT_NOT]], label [[EXIT:%.*]], label [[LOOP]]
 ; CHECK:       exit:
@@ -92,7 +92,7 @@ define i4 @iv_hoist_both_adds_nsw_extra_use(i4 %arg) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i4 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw nsw i4 [[IV_0]], 1
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i4 1, [[ARG:%.*]]
@@ -124,7 +124,7 @@ define i4 @iv_hoist_both_adds_nsw_extra_use_incs_reordered(i4 %arg) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i4 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw nsw i4 [[IV_0]], 1
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i4 1, [[ARG:%.*]]
@@ -244,7 +244,7 @@ define i2 @iv_hoist_both_adds_nuw(i2 %arg, i2 %start) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i2 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add i2 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i2 [[IV_0]], 1
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i2 [[START]], [[ARG:%.*]]
 ; CHECK-NEXT:    br i1 [[DOTNOT_NOT]], label [[EXIT:%.*]], label [[LOOP]]
 ; CHECK:       exit:
@@ -272,7 +272,7 @@ define i4 @iv_hoist_both_adds_nuw_extra_use(i4 %arg, i4 %start) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i4 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add i4 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i4 [[START]], [[ARG:%.*]]
@@ -304,7 +304,7 @@ define i4 @iv_hoist_both_adds_nuw_extra_use_incs_reordered(i4 %arg, i4 %start) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV_0:%.*]] = phi i4 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[IV_0_NEXT]] = add i4 [[IV_0]], 1
+; CHECK-NEXT:    [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    call void @use(i4 [[IV_0_NEXT]])
 ; CHECK-NEXT:    [[DOTNOT_NOT:%.*]] = icmp ult i4 [[START]], [[ARG:%.*]]