https://llvm.org/bugs/show_bug.cgi?id=26159
Bug ID: 26159
Summary: LLVM disregards windows’ x64 vararg ABI
Product: new-bugs
Version: 3.7
Hardware: PC
OS: other
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: simonas+llvm....@kazlauskas.me
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
LLVM will reliably mis-translate code calling vararg-using functions which
targets Windows x64.
x64 ABI requires no special handling to varargs as compared to non-vararg
functions and wants some varargs to be placed into registers, not always stack.
LLVM, though, always places all of the varargs onto stack.
Here’s a few C programs that, as compiled with clang 3.7.1 on Windows x64,
produce unexpected results:
#include<stdio.h>
extern int wsprintfA(char *out, const char* in, ...);
int main() {
char out[100];
wsprintfA(out, "%d %f %c", 42, 42.25, 'a');
printf("%s", out); // prints “42 f”
return 0;
}
---
extern int wsprintfA(char *out, const char* in, ...);
int main() {
char out[100];
wsprintfA(out, "%d %f %c %s", 42, 42.25, 'a', "hello world"); //segfaults
return 0;
}
(you can think of wsprintfA as of alias to sprintf, because it [essentially is
one](https://msdn.microsoft.com/en-us/library/windows/desktop/ms647550(v=vs.85).aspx))
I have verified that LLVM generated code correctly places the non-vararg
arguments into the correct registers, however it fails to do so for the
vararg-arguments.
---
I also have a minimal LLVM-IR file which reproduces the issue, but is not as
trivial to build.
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