Answer to va_list on Microsoft’s x64

A while back I wrote about bad va_list assumptions. Recap: AMD64 passes some arguments through registers, and GNU’s va_list structure changes to accomodate that. Such changes mean you need to use va_copy instead of relying on x86 assumptions.

Microsoft does not have va_copy, so I was unsure how their x64 compiler solved the problem. I had three guesses: 1) va_list could be copied through assignment, 2) all variadic functions required every parameter to be on the stack, or 3) something else.

It turned out to be something else. Microsoft takes a rather strange approach. The caller reserves space on the stack for all of the registers who have arguments being passed. Then it moves the data into the respective registers but doesn’t touch the stack. The variadic callee then moves these register values into the stack above the frame, that is, where the other variadic parameters are.

For example, here is how logmessage() gets called:

Select All Code:
000000013F7F1060  sub         rsp,28h 
    logmessage("%s %s %s\n", "a", "b", "c");
000000013F7F1064  lea         r9,[string "c" (13F7F21B0h)] 
000000013F7F106B  lea         r8,[string "b" (13F7F21B4h)] 
000000013F7F1072  lea         rdx,[string "a" (13F7F21B8h)] 
000000013F7F1079  lea         rcx,[string "%s %s %s\n" (13F7F21C0h)] 
000000013F7F1080  call        logmessage (13F7F1000h)

And, here is logmessage()‘s prologue, which immediately saves its four arguments in the stack space above its frame.

Select All Code:
void logmessage(const char *fmt, ...)
000000013F7F1000  mov         qword ptr [rsp+8],rcx 
000000013F7F1005  mov         qword ptr [rsp+10h],rdx 
000000013F7F100A  mov         qword ptr [rsp+18h],r8 
000000013F7F100F  mov         qword ptr [rsp+20h],r9

After doing that, the register complication of AMD64 is removed, because everything just sits on the stack. Thus the va_list variable can be re-used because it’s just a by-value pointer to the stack:

Select All Code:
    va_start(ap, fmt);
000000013F7F1019  lea         rbx,[rsp+38h] 
    vfprintf(stdout, fmt, ap);
000000013F7F101E  call        qword ptr [__imp___iob_func (13F7F2138h)]

And indeed, it appears to work fine:

a b c
a b c
Press any key to continue . . .

This implementation is interesting to me and I’d love to know the reasoning behind it. I have one big guess: it preserves the calling convention. The other option is to say, “all variadic functions must pass everything on the stack.” Perhaps that additional bit of complexity was undesired, or perhaps there are optimization cases where you’d want variadic functions that don’t immediately use the stack or va_list, but still need CRT compatibility.

Whatever the case, it’s not a big deal.

And, if you were wondering: You can indeed assign va_list pointers on Microsoft’s x64 compiler. GNU forbids that so I’m unsure if that’s intended or an accident on Microsoft’s part.

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