https://llvm.org/bugs/show_bug.cgi?id=27209
Bug ID: 27209
Summary: failure to optimize a simple loop to a memset call
(incorrect fear of aliasing?)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: richard-l...@metafoo.co.uk
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
LLVM fails to optimize this simple loop to a memset:
void f(char *p, char *q) {
for (char *r = q; r != q + 256; ++r) *r = *p;
}
Presumably the relevant optimization is not firing here because of fear that p
might alias some element of the array pointed at by q. However, that doesn't
matter, because *p is the only thing written to that array, and the store size
is the same as the alignment, so *p can never actually change as a result.
See also https://llvm.org/bugs/show_bug.cgi?id=27206 -- LLVM should at least be
able to optimize that if the type of the field within struct S is char, and
should be able to hoist the load of s.i out of the loop for any type where size
== alignment. (And using C++ object lifetime rules, the hoisting is always
permitted regardless of alignment.)
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