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9baaae9344
Fixes PR27315.
The post-inc version of an add recurrence needs to "follow the same
rules" as a normal add or subtract expression. Otherwise we miscompile
programs like
```
int main() {
int a = 0;
unsigned a_u = 0;
volatile long last_value;
do {
a_u += 3;
last_value = (long) ((int) a_u);
if (will_add_overflow(a, 3)) {
// Leave, and don't actually do the increment, so no UB.
printf("last_value = %ld\n", last_value);
exit(0);
}
a += 3;
} while (a != 46);
return 0;
}
```
This patch changes SCEV to put no-wrap flags on post-inc add recurrences
only when the poison from a potential overflow will go ahead to cause
undefined behavior.
To avoid regressing performance too much, I've assumed infinite loops
without side effects is undefined behavior to prove poison<->UB
equivalence in more cases. This isn't ideal, but is not new to LLVM as
a whole, and far better than the situation I'm trying to fix.
llvm-svn: 271151
32 lines
901 B
LLVM
32 lines
901 B
LLVM
; RUN: opt -analyze -scalar-evolution < %s | FileCheck %s
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declare i1 @use(i64)
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define void @f_0() {
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; CHECK-LABEL: Classifying expressions for: @f_0
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; CHECK: %iv = phi i32 [ 0, %entry ], [ %iv.inc.nowrap, %be ]
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; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%loop>
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; CHECK: %iv.inc.maywrap = add i32 %iv, 1
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; CHECK-NEXT: --> {1,+,1}<nuw><%loop>
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; CHECK: %iv.inc.maywrap.sext = sext i32 %iv.inc.maywrap to i64
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; CHECK-NEXT: --> (sext i32 {1,+,1}<nuw><%loop> to i64)
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.inc.nowrap, %be ]
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%iv.inc.maywrap = add i32 %iv, 1
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%iv.inc.maywrap.sext = sext i32 %iv.inc.maywrap to i64
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%cond0 = call i1 @use(i64 %iv.inc.maywrap.sext)
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br i1 %cond0, label %be, label %leave
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be:
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%iv.inc.nowrap = add nsw i32 %iv, 1
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%be.cond = call i1 @use(i64 0) ;; Get an unanalyzable value
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br i1 %be.cond, label %loop, label %leave
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leave:
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ret void
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}
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