[PATCH] Bugfix: SCEV incorrectly marks certain add recurrences as nsw
Sanjoy Das
sanjoy at playingwithpointers.com
Sun Feb 8 19:27:04 PST 2015
Hi atrick, hfinkel, majnemer, nlewycky,
When creating a scev for sext({X,+,Y}), scev checks if the expression
is equivalent to {sext X,+,zext Y}. If it can prove that, it also
tags the original {X,+,Y} as <nsw>, which is not correct AFAICT.
In the test case I run `-scalar-evolution` twice because the bug
manifests only once SCEV has run through and seen the `sext`
expressions (and then does a in-place mutation on {X,+,Y}).
http://reviews.llvm.org/D7495
Files:
lib/Analysis/ScalarEvolution.cpp
test/Analysis/ScalarEvolution/incorrect-nsw.ll
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -1550,8 +1550,24 @@
getMulExpr(WideMaxBECount,
getZeroExtendExpr(Step, WideTy)));
if (SAdd == OperandExtendedAdd) {
- // Cache knowledge of AR NSW, which is propagated to this AddRec.
- const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNSW);
+ if (isKnownNonZero(Step)) {
+ // If AR wraps around then
+ //
+ // (Start + I * Step) == Start <=> I * Step == 0
+ //
+ // Assuming Step != 0, there are two possibilities:
+ //
+ // 1. I == 0: this means that the backedge is never taken, and
+ // hence AR can be marked NW.
+ //
+ // 2. I * Step unsigned-overflowed: this means SAdd !=
+ // OperandExtendedAdd
+ //
+ // Thus (AR is not NW => SAdd != OperandExtendedAdd) <=>
+ // (SAdd == OperandExtendedAdd => AR is NW)
+ //
+ const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNW);
+ }
// Return the expression with the addrec on the outside.
return getAddRecExpr(getSignExtendAddRecStart(AR, Ty, this),
getZeroExtendExpr(Step, Ty),
Index: test/Analysis/ScalarEvolution/incorrect-nsw.ll
===================================================================
--- /dev/null
+++ test/Analysis/ScalarEvolution/incorrect-nsw.ll
@@ -0,0 +1,26 @@
+; RUN: opt -analyze -scalar-evolution -scalar-evolution < %s | FileCheck %s
+
+define void @bad.nsw() {
+; CHECK-LABEL: Classifying expressions for: @bad.nsw
+; CHECK-LABEL: Classifying expressions for: @bad.nsw
+ entry:
+ br label %loop
+
+ loop:
+ %i = phi i8 [ -1, %entry ], [ %i.inc, %loop ]
+; CHECK: %i = phi i8 [ -1, %entry ], [ %i.inc, %loop ]
+; CHECK-NEXT: --> {-1,+,-128}<nw><%loop>
+; CHECK-NOT: --> {-1,+,-128}<nsw><%loop>
+
+ %counter = phi i8 [ 0, %entry ], [ %counter.inc, %loop ]
+
+ %i.inc = add i8 %i, -128
+ %i.sext = sext i8 %i to i16
+
+ %counter.inc = add i8 %counter, 1
+ %continue = icmp eq i8 %counter, 1
+ br i1 %continue, label %exit, label %loop
+
+ exit:
+ ret void
+}
\ No newline at end of file
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