Re: AMD64 ABI and comparison of structures

[Andrew Haley <aph at redhat dot com>]

José Bollo writes:
 > hi all,
 > 
 > I repost here a problem that I met using GCC 3.4.4 and ABI for AMD64 
 > (-march=athlon64).
 > 
 > the following C code 
 > 
 > typedef struct {
 >     int32_t a;
 >     void *b;
 > } S;
 > void fs() {
 >     char t[255];
 >     int i;
 >     for(i=0;i<255;i++)
 >         t[i]=(char)(i+1);
 > }
 > int cmp1(S a1, S a2) {
 >     return !memcmp(&a1,&a2,sizeof(S));
 > }
 > int cmp2(S a1, S a2) {
 >     return a1.a==a2.a && a1.b==a2.b;
 > }
 > void put(S y) {
 >     int a,b;
 >     S x;
 >     x=y;
 >     a=cmp1(x,y);
 >     b=cmp2(x,y);
 >     printf("a=%d b=%d\n",a,b);
 > }
 > int main() {
 >     S a;
 >     a.a=1;
 >     a.b=&a;
 >     fs();
 >     put(a);
 >     return 0;
 > }
 > 
 > compiled for my linux x86_64 using the amd64 ABI (-march=athlon64) with gcc 
 > 3.4.4 prints "a=0 b=1"!
 > 
 > i have seen why and it is clear to me how a such result is produced.
 > I add below a commented assembly listing of the incriminated part.
 > 
 > i think that it is not a bug and that the structure can not be compared using 
 > 'memcmp'.

That's right.  It's a consequence of 6.7.2.1 Para 13: "There may be
unnamed padding within a structure object, but not at its beginning".

Andrew.