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Add back some possible optimizations for va_arg, with wording that makes it 

more clear what exactly is missing.

llvm-svn: 105934
This commit is contained in:
Eli Friedman 2010-06-14 07:03:30 +00:00
parent 4f475d07ba
commit ba1f1fcae5
1 changed files with 31 additions and 0 deletions
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31
llvm/lib/Target/X86/README-X86-64.txt
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@ -240,3 +240,34 @@ Issues:
expensive addressing mode.
//===---------------------------------------------------------------------===//
Consider the following (contrived testcase, but contains common factors):
#include <stdarg.h>
int test(int x, ...) {
int sum, i;
va_list l;
va_start(l, x);
for (i = 0; i < x; i++)
sum += va_arg(l, int);
va_end(l);
return sum;
}
Testcase given in C because fixing it will likely involve changing the IR
generated for it. The primary issue with the result is that it doesn't do any
of the optimizations which are possible if we know the address of a va_list
in the current function is never taken:
1. We shouldn't spill the XMM registers because we only call va_arg with "int".
2. It would be nice if we could scalarrepl the va_list.
3. Probably overkill, but it'd be cool if we could peel off the first five
iterations of the loop.
Other optimizations involving functions which use va_arg on floats which don't
have the address of a va_list taken:
1. Conversely to the above, we shouldn't spill general registers if we only
call va_arg on "double".
2. If we know nothing more than 64 bits wide is read from the XMM registers,
we can change the spilling code to reduce the amount of stack used by half.
//===---------------------------------------------------------------------===//