[llvm-commits] [PATCH]: ppc32 va_arg() implementation

[Jim Grosbach]

On Jun 14, 2011, at 1:11 PM, Roman Divacky wrote:

> On Tue, Jun 14, 2011 at 01:09:07PM -0700, Eli Friedman wrote:
>> On Tue, Jun 14, 2011 at 1:02 PM, Roman Divacky <rdivacky at freebsd.org> wrote:
>>> On Tue, Jun 14, 2011 at 12:59:18PM -0700, Eli Friedman wrote:
>>>> On Tue, Jun 14, 2011 at 12:47 PM, Roman Divacky <rdivacky at freebsd.org> wrote:
>>>>> Hi,
>>>>> 
>>>>> the attached patch implements va_arg() lowering on ppc32.
>>>>> Ints (8, 16, 32 and also 64 bits that need special treatment) work,
>>>>> floats don't work (neither with gcc - we should warn when someone tries
>>>>> to use them on ppc32 probably), doubles do work.
>>>> 
>>>> It's impossible to make any C frontend generate a call to a varargs
>>>> function that actually passes in something of type "float"... are you
>>>> sure you're testing correctly?
>>> 
>>> This is an example with doubles, to change it to floats you just
>>> change the type and the literal passed in. Am I doing something wrong?:
>>> 
>>> void bar(int w, ...) {
>>> ?double d;
>>> ?va_list ap;
>>> ?va_start(ap, w);
>>> 
>>> ?printf("\n\n");
>>> ?do {
>>> ? ?d = va_arg(ap, double);
>>> ? ?printf("d = %f\n", d);
>>> ?} while (d != 0.0);
>>> ?va_end(ap);
>>> }
>>> 
>>> int main() {
>>> ?bar(1, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 0.0);
>>> }
>>> 
>> 
>> Changing the type of the literal passed in doesn't change the
>> generated code; they get promoted them to doubles.
> 
> Yes, but when I use va_arg(ap, float) it does not work (crashes gcc, generates
> the wrong result with llvm+my patch).

Eli is correct. Any floating point argument that's part of a variable length argument list is always passed as a double. This is part of the C standard. If you're accessing it as a float, you're not going to get correct results on any platform where float and double are of different size.

-Jim