const SCEVAddRecExpr *PreAR = dyn_cast<SCEVAddRecExpr>(
SE->getAddRecExpr(PreStart, Step, L, SCEV::FlagAnyWrap));
- if (PreAR && PreAR->getNoWrapFlags(SCEV::FlagNSW))
+ // WARNING: FIXME: the optimization below assumes that a sign-overflowing nsw
+ // 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 arithmetic through control flow). This logic
+ // needs to be revisited once we have a consistent semantics for poison
+ // values.
+ //
+ // "{S,+,X} is <nsw>" and "{S,+,X} is evaluated at least once" implies "S+X
+ // does not sign-overflow" (we'd have undefined behavior if it did). If
+ // `L->getExitingBlock() == L->getLoopLatch()` then `PreAR` (= {S,+,X}<nsw>)
+ // 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-overflow.
+ //
+
+ BasicBlock *ExitingBlock = L->getExitingBlock();
+ BasicBlock *LatchBlock = L->getLoopLatch();
+ if (PreAR && PreAR->getNoWrapFlags(SCEV::FlagNSW) &&
+ ExitingBlock != nullptr && ExitingBlock == LatchBlock)
return PreStart;
// 2. Direct overflow check on the step operation's expression.
--- /dev/null
+; RUN: opt -analyze -scalar-evolution < %s | FileCheck %s
+
+; An example run where SCEV(%postinc)->getStart() may overflow:
+;
+; %start = INT_SMAX
+; %low.limit = INT_SMIN
+; %high.limit = < not used >
+;
+; >> entry:
+; %postinc.start = INT_SMIN
+;
+; >> loop:
+; %idx = %start
+; %postinc = INT_SMIN
+; %postinc.inc = INT_SMIN + 1
+; %postinc.sext = sext(INT_SMIN) = i64 INT32_SMIN
+; %break.early = INT_SMIN `slt` INT_SMIN = false
+; br i1 false, ___, %early.exit
+;
+; >> early.exit:
+; ret i64 INT32_SMIN
+
+
+define i64 @bad.0(i32 %start, i32 %low.limit, i32 %high.limit) {
+; CHECK-LABEL: Classifying expressions for: @bad.0
+ entry:
+ %postinc.start = add i32 %start, 1
+ br label %loop
+
+ loop:
+ %idx = phi i32 [ %start, %entry ], [ %idx.inc, %continue ]
+ %postinc = phi i32 [ %postinc.start, %entry ], [ %postinc.inc, %continue ]
+ %postinc.inc = add nsw i32 %postinc, 1
+ %postinc.sext = sext i32 %postinc to i64
+; CHECK: %postinc.sext = sext i32 %postinc to i64
+; CHECK-NEXT: --> {(sext i32 (1 + %start) to i64),+,1}<nsw><%loop>
+ %break.early = icmp slt i32 %postinc, %low.limit
+ br i1 %break.early, label %continue, label %early.exit
+
+ continue:
+ %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
+}
+
+define i64 @bad.1(i32 %start, i32 %low.limit, i32 %high.limit, i1* %unknown) {
+; CHECK-LABEL: Classifying expressions for: @bad.1
+ entry:
+ %postinc.start = add i32 %start, 1
+ br label %loop
+
+ loop:
+ %idx = phi i32 [ %start, %entry ], [ %idx.inc, %continue ], [ %idx.inc, %continue.1 ]
+ %postinc = phi i32 [ %postinc.start, %entry ], [ %postinc.inc, %continue ], [ %postinc.inc, %continue.1 ]
+ %postinc.inc = add nsw i32 %postinc, 1
+ %postinc.sext = sext i32 %postinc to i64
+; CHECK: %postinc.sext = sext i32 %postinc to i64
+; CHECK-NEXT: --> {(sext i32 (1 + %start) to i64),+,1}<nsw><%loop>
+ %break.early = icmp slt i32 %postinc, %low.limit
+ br i1 %break.early, label %continue.1, label %early.exit
+
+ continue.1:
+ %cond = load volatile i1* %unknown
+ %idx.inc = add nsw i32 %idx, 1
+ br i1 %cond, label %loop, label %continue
+
+ continue:
+ %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
+}
+
+
+; 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
+}