mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-12-13 20:32:21 +00:00
Bugfix: ScalarEvolution incorrectly assumes that the start of certain
add recurrences don't overflow. This change makes the optimization more restrictive. It still assumes that an overflowing `add nsw` is undefined behavior; and this change will need revisiting once we have a consistent semantics for poison values. Differential Revision: http://reviews.llvm.org/D7331 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228552 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das
parent
3824fd3a25
commit
1af442e572
@ -1364,7 +1364,24 @@ static const SCEV *getPreStartForSignExtend(const SCEVAddRecExpr *AR,
|
||||
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.
|
||||
|
||||
116
test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll
Normal file
116
test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll
Normal file
@ -0,0 +1,116 @@
|
||||
; 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
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user