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

[Nikita Popov via Phabricator via llvm-commits]

nikic updated this revision to Diff 198288.
nikic added a comment.

Use both pre-inc and post-inc AddRec nowrap flags.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D60935/new/

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:
@@ -117,6 +117,33 @@
   ret i32 0
 }
 
+define i32 @test_no_add_nuw() {
+; CHECK-LABEL: @test_no_add_nuw(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], 10
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  br label %loop
+
+loop:
+  %storemerge = phi i32 [ -1, %entry ], [ %inc, %loop ]
+  store i32 %storemerge, i32* @a
+  %cmp = icmp slt i32 %storemerge, 9
+  %inc = add i32 %storemerge, 1
+  br i1 %cmp, label %loop, label %exit
+
+exit:
+  ret i32 0
+}
+
 define i32 @test_drop_nsw_var_lim(i32 %lim) {
 ; CHECK-LABEL: @test_drop_nsw_var_lim(
 ; CHECK-NEXT:  entry:
@@ -128,7 +155,7 @@
 ; 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 +2375,23 @@
     // 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());
+
+    // It may be that the result of the increment is poison (due to nuw/nsw
+    // flags) on the last iterations only, in which case moving the comparison
+    // after the increment may invalidate the nuw/nsw flags. We combine the
+    // computed SCEV nowrap flags from the pre-increment and the post-increment
+    // AddRecs. The former covers all but the last iteration, while the former
+    // covers all but the first. If a nowrap flag is set on both, it implies
+    // that the operation is non-wrapping for all iterations.
+    if (auto *BO = dyn_cast<BinaryOperator>(CmpIndVar)) {
+      const SCEVAddRecExpr *PreAR = cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
+      const SCEVAddRecExpr *PostAR =
+          cast<SCEVAddRecExpr>(SE->getSCEV(CmpIndVar));
+      BO->setHasNoUnsignedWrap(
+          PreAR->hasNoUnsignedWrap() && PostAR->hasNoUnsignedWrap());
+      BO->setHasNoSignedWrap(
+          PreAR->hasNoSignedWrap() && PostAR->hasNoSignedWrap());
+    }
   }
 
   Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);


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