While I haven't heard back from you about my response re volatile, I'm
going to go ahead and write my post about why I think my understanding
of the memory clobber is correct. Should give you plenty to think about
over the weekend.
In my original post, I said that I wanted to add the memory clobber to
the __sto* routines. Your response was "Please don't do that."
Now, if my understanding of how the memory clobber works is correct,
this provides an excellent opportunity to prove it. Consider this code
(__stosb copied from existing winnt.h):
#include <stdio.h>
#include <assert.h>
typedef unsigned char BYTE;
typedef unsigned char *PBYTE;
typedef unsigned long long SIZE_T;
typedef void VOID;
__CRT_INLINE VOID __stosb(PBYTE Dest,BYTE Data,SIZE_T Count)
{
__asm__ __volatile__
(
"rep; stosb" :
[Dest] "=D" (Dest), [Count] "=c" (Count) :
"[Dest]" (Dest), "a" (Data), "[Count]" (Count)
);
}
int main(int argc, char* argv[])
{
struct
{
int a;
int b;
} c;
c.a = atoi(argv[1]);
c.b = atoi(argv[2]);
__stosb((PBYTE)&c, 0, sizeof(c));
printf("%u %u\n", c.a, c.b);
}
If I compile this with:
c++.exe -o %1.exe -march=native -mwin32 -Os -m64 %1.cpp
and execute it with the line:
n 1 2
What result would you expect to get printed?
a) 0, 0
b) 0, 2
c) 1, 2
Without the memory clobber (your solution), you get (b). With the
memory clobber (my solution), you get (a). I believe (a) is correct.
This demonstrates the errors you get if you omit the "memory" clobber
when it is required. If I get a chance, I'll create a second sample
that shows the performance penalty you pay for using it when it isn't
necessary.
dw
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