[PATCH] D60935: [IndVarSimplify] Fixup nowrap flags during LFTR when moving to post-inc (PR31181)

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: mkazantsev, sanjoy, hans.
Herald added subscribers: llvm-commits, javed.absar, hiraditya.
Herald added a project: LLVM.

Fix for https://bugs.llvm.org/show_bug.cgi?id=31181. When LFTR moves a condition prior to increment to a condition after increment, it may depend on a value that is now poison due to nowrap flags. We need to make sure to drop the nowrap flags on the add in such a situation. I'm doing this by copying the nowrap flags that SCEV computed.

Once this issue is fixed, we can reenable computation of nowrap flags in CVP.


Repository:
  rL LLVM

https://reviews.llvm.org/D60935

Files:
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
  llvm/test/Transforms/IndVarSimplify/pr31181.ll


Index: llvm/test/Transforms/IndVarSimplify/pr31181.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/pr31181.ll
+++ llvm/test/Transforms/IndVarSimplify/pr31181.ll
@@ -15,7 +15,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ -2, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], 0
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
@@ -42,7 +42,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nuw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], -2147483648
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
@@ -128,7 +128,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ [[INC:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nuw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], [[TMP0]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
 ; CHECK:       exit.loopexit:
Index: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -2375,6 +2375,15 @@
     // backedge branches to the loop header.  This is one less than the
     // number of times the loop executes, so use the incremented indvar.
     CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock());
+
+    // The result of the increment may be poison due to nuw/nsw flags. SCEV
+    // already keeps track of the correct nuw/nsw flags, so we just copy
+    // whatever it computed here.
+    if (auto *BO = dyn_cast<BinaryOperator>(CmpIndVar)) {
+      const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(CmpIndVar));
+      BO->setHasNoUnsignedWrap(AR->hasNoUnsignedWrap());
+      BO->setHasNoSignedWrap(AR->hasNoSignedWrap());
+    }
   }
 
   Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);


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