https://bugs.llvm.org/show_bug.cgi?id=51794
Bug ID: 51794
Summary: [LoopVectorizer] Suspected wrong code at -Os
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
This program:
int a, d = 0 <= 0;
int *b = &a;
short c = 5;
int main() {
*b = d;
e:
--c;
*b &= b != 0;
*b ^= -1L;
if (c)
goto e;
printf("%d\n", a);
}
should print '-2', which it does except at -Os when it prints '0' (clang
-march=arch13 -O2 wrong0).
If the loop vectorizer is disabled, -2 is printed also at -Os.
Given that the loop is obviously executed 5 times I suspect that the check
whether to go into the vectorized loop or scalarized loop is wrong at -Os.
At -O2 this check seems correct:
%c.promoted = load i16, i16* @c, align 2, !tbaa !8
%2 = add i16 %c.promoted, -1
%3 = zext i16 %2 to i32
%4 = add nuw nsw i32 %3, 1
%min.iters.check = icmp ult i32 %4, 16
br i1 %min.iters.check, label %scalar.ph, label %vector.ph
This should mean that in this case only the scalar loop is executed.
However at -Os I see this instead:
%c.promoted = load i16, i16* @c, align 2, !tbaa !8
%2 = add i16 %c.promoted, -1
%3 = zext i16 %2 to i32
%4 = add nuw nsw i32 %3, 1
br i1 false, label %scalar.ph, label %vector.ph
For some reason the vectorized loop is now always executed, which in this case
should not happen as it will then run more times than intended (at least 16).
This seems strange as the result after iterations are not constant but are
rather toggling and ending up at -2 after exactly 5 iterations, if I am not
mistaken...
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