http://llvm.org/bugs/show_bug.cgi?id=11197
Summary: varargs is unnecessarily inefficient
Product: libraries
Version: trunk
Platform: All
OS/Version: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
The way LLVM implements varargs is unnecessarily inefficient. It seems to force
the "va_list" variable into memory when it could easily be mapped to a
register. Is there a way to circumvent this behavior?
Consider the following test case:
#include <stdarg.h>
void foo(int *p, ...){
va_list ap;
va_start(ap,p);
while(1){
*p = va_arg(ap,int);
if (*p == 0) break;
p++;
}
}
The ARM target compiles this (at -O3) such that the loop code is quite strange:
.LBB0_3:
ldr r2, [sp] <--- Unnecessary
add r0, r0, #4
add r1, r2, #4
str r1, [sp] <---- Unnecessary
ldr r2, [r2]
str r2, [r0]
cmp r2, #0
bne .LBB0_3
By contrast, GCC produces a loop that is half the size without the silly load
and store of the va_list variable:
.L3:
ldr r0, [r1, #4]!
cmp r0, #0
str r0, [r2, #4]!
bne .L3
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