https://llvm.org/bugs/show_bug.cgi?id=26997
Bug ID: 26997
Summary: memset() is optimised out even if followed by
asm("":::"memory")
Product: clang
Version: 3.7
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: socketp...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
suppose program:
=============================
#include <string.h>
int main() {
char buf[256];
memset(buf,0, sizeof(buf));
asm volatile("": : :"memory");
return buf[0];
}
=============================
clang-3.7 -O3 throw out memset() call. (gcc 5.2.1 does not).
if I change assembler directive to
asm volatile("": : "g" (buf) :"memory");
everything works right. AFAIK, clobbering memory mean clobbering ALL memory, so
this may mean that any memory (including buf array) has been potentialy
changed. Since I return first byte of that array, clang (llvm?) should not
think that this byte is always zero.
Interestingly, but
asm volatile("nop": : :"memory");
will elimiate memset() anyway.
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