Fix bug 22641

The bug was a result of getPreStartForExtend interpreting nsw/nuw
flags on an add recurrence more strongly than is legal.  {S,+,X}<nsw>
implies S+X is nsw only if the backedge of the loop is taken at least
once.

NOTE: I had accidentally committed an unrelated change with the commit
message of this change in r230275 (r230275 was reverted in r230279).
This is the correct change for this commit message.

Differential Revision: http://reviews.llvm.org/D7808

llvm-svn: 230291

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 22e361c..9e4eb11 100644 (file)
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -1274,25 +1274,13 @@ static const SCEV *getPreStartForExtend(const SCEVAddRecExpr *AR, Type *Ty,
   const SCEVAddRecExpr *PreAR = dyn_cast<SCEVAddRecExpr>(
       SE->getAddRecExpr(PreStart, Step, L, SCEV::FlagAnyWrap));
 
-  // WARNING: FIXME: the optimization below assumes that a sign/zero-overflowing
-  // nsw/nuw operation is undefined behavior.  This is strictly more aggressive
-  // than the interpretation of nsw in other parts of LLVM (for instance, they
-  // may unconditionally hoist nsw/nuw arithmetic through control flow).  This
-  // logic needs to be revisited once we have a consistent semantics for poison
-  // values.
-  //
-  // "{S,+,X} is <nsw>/<nuw>" and "{S,+,X} is evaluated at least once" implies
-  // "S+X does not sign/unsign-overflow" (we'd have undefined behavior if it
-  // did).  If `L->getExitingBlock() == L->getLoopLatch()` then `PreAR` (=
-  // {S,+,X}<nsw>/<nuw>) is evaluated every-time `AR` (= {S+X,+,X}) is
-  // evaluated, and hence within `AR` we are safe to assume that "S+X" will not
-  // sign/unsign-overflow.
+  // "{S,+,X} is <nsw>/<nuw>" and "the backedge is taken at least once" implies
+  // "S+X does not sign/unsign-overflow".
   //
 
-  BasicBlock *ExitingBlock = L->getExitingBlock();
-  BasicBlock *LatchBlock = L->getLoopLatch();
-  if (PreAR && PreAR->getNoWrapFlags(WrapType) && ExitingBlock != nullptr &&
-      ExitingBlock == LatchBlock)
+  const SCEV *BECount = SE->getBackedgeTakenCount(L);
+  if (PreAR && PreAR->getNoWrapFlags(WrapType) &&
+      !isa<SCEVCouldNotCompute>(BECount) && SE->isKnownPositive(BECount))
     return PreStart;
 
   // 2. Direct overflow check on the step operation's expression.

diff --git a/llvm/test/Analysis/ScalarEvolution/infer-prestart-no-wrap.ll b/llvm/test/Analysis/ScalarEvolution/infer-prestart-no-wrap.ll
index 45b3e4c..c9689f7 100644 (file)
--- a/llvm/test/Analysis/ScalarEvolution/infer-prestart-no-wrap.ll
+++ b/llvm/test/Analysis/ScalarEvolution/infer-prestart-no-wrap.ll
@@ -6,12 +6,14 @@ define void @infer.sext.0(i1* %c, i32 %start) {
   br label %loop
 
  loop:
+  %counter = phi i32 [ 0, %entry ], [ %counter.inc, %loop ]
   %idx = phi i32 [ %start, %entry ], [ %idx.inc, %loop ]
   %idx.inc = add nsw i32 %idx, 1
   %idx.inc.sext = sext i32 %idx.inc to i64
 ; CHECK: %idx.inc.sext = sext i32 %idx.inc to i64
 ; CHECK-NEXT: -->  {(1 + (sext i32 %start to i64)),+,1}<nsw><%loop>
-  %condition = load volatile i1* %c
+  %condition = icmp eq i32 %counter, 1
+  %counter.inc = add i32 %counter, 1
   br i1 %condition, label %exit, label %loop
 
  exit:
@@ -24,12 +26,14 @@ define void @infer.zext.0(i1* %c, i32 %start) {
   br label %loop
 
  loop:
+  %counter = phi i32 [ 0, %entry ], [ %counter.inc, %loop ]
   %idx = phi i32 [ %start, %entry ], [ %idx.inc, %loop ]
   %idx.inc = add nuw i32 %idx, 1
   %idx.inc.sext = zext i32 %idx.inc to i64
 ; CHECK: %idx.inc.sext = zext i32 %idx.inc to i64
 ; CHECK-NEXT: -->  {(1 + (zext i32 %start to i64)),+,1}<nuw><%loop>
-  %condition = load volatile i1* %c
+  %condition = icmp eq i32 %counter, 1
+  %counter.inc = add i32 %counter, 1
   br i1 %condition, label %exit, label %loop
 
  exit:

diff --git a/llvm/test/Analysis/ScalarEvolution/pr22641.ll b/llvm/test/Analysis/ScalarEvolution/pr22641.ll
new file mode 100644 (file)
index 0000000..3b55afe
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/pr22641.ll
@@ -0,0 +1,25 @@
+; RUN: opt -analyze -scalar-evolution < %s | FileCheck %s
+
+define i1 @main(i16 %a) {
+; CHECK-LABEL: Classifying expressions for: @main
+entry:
+  br label %body
+
+body:
+  %dec2 = phi i16 [ %a, %entry ], [ %dec, %cond ]
+  %dec = add i16 %dec2, -1
+  %conv2 = zext i16 %dec2 to i32
+  %conv = zext i16 %dec to i32
+; CHECK:   %conv = zext i16 %dec to i32
+; CHECK-NEXT: -->  {(zext i16 (-1 + %a) to i32),+,65535}<nuw><%body>
+; CHECK-NOT:  -->  {(65535 + (zext i16 %a to i32)),+,65535}<nuw><%body>
+
+  br label %cond
+
+cond:
+  br i1 false, label %body, label %exit
+
+exit:
+  %ret = icmp ne i32 %conv, 0
+  ret i1 %ret
+}

diff --git a/llvm/test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll b/llvm/test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll
index 5dbb6e6..3ca32bd 100644 (file)
--- a/llvm/test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll
+++ b/llvm/test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll
@@ -80,37 +80,3 @@ define i64 @bad.1(i32 %start, i32 %low.limit, i32 %high.limit, i1* %unknown) {
  early.exit:
   ret i64 %postinc.sext
 }
-
-
-; WARNING: FIXME: it is safe to make the inference demonstrated here
-; only if we assume `add nsw` has undefined behavior if the result
-; sign-overflows; and this interpretation is stronger than what most
-; of LLVM assumes.  This test here only serves as a documentation of
-; current behavior and will need to be revisited once we've decided
-; upon a consistent semantics for nsw (and nuw) arithetic operations.
-;
-define i64 @good(i32 %start, i32 %low.limit, i32 %high.limit) {
-; CHECK-LABEL: Classifying expressions for: @good
- entry:
-  %postinc.start = add i32 %start, 1
-  br label %loop
-
- loop:
-  %idx = phi i32 [ %start, %entry ], [ %idx.inc, %loop ]
-  %postinc = phi i32 [ %postinc.start, %entry ], [ %postinc.inc, %loop ]
-  %postinc.inc = add nsw i32 %postinc, 1
-  %postinc.sext = sext i32 %postinc to i64
-; CHECK: %postinc.sext = sext i32 %postinc to i64
-; CHECK-NEXT: -->  {(1 + (sext i32 %start to i64)),+,1}<nsw><%loop>
-
-  %break.early = icmp slt i32 %postinc, %low.limit
-  %idx.inc = add nsw i32 %idx, 1
-  %cmp = icmp slt i32 %idx.inc, %high.limit
-  br i1 %cmp, label %loop, label %exit
-
- exit:
-  ret i64 0
-
- early.exit:
-  ret i64 %postinc.sext
-}