🚀 go-pugleaf

Hi,

Here is an optimization gcc could perform in its pointer and aliasing
analysis:

If p is a pointer to a (non-volatile) word in memory, and a copy of this word
is also in a register, then writing this word out to somewhere in memory does
_not_ make the register copy of *p invalid, assuming that alignof(word) ==
sizeof(word). (Because if in the assignment "*q = *p", q != p, then *p wasn't
affected, while if q == p, it was overwritten with itself.)

Here is a test case which shows a superfluous memory fetch:

========================== foo.c ===============================
typedef struct mylist {
	struct bfd * abfd;
	struct mylist * next;
	struct mylist ** prev;
} mylist;
void mylist_insert (struct mylist ** p, struct mylist * n)
{
	n->next = *p;
	if (*p)
		(*p)->prev = &n->next;
	n->prev = p;
	*p = n;
}
========================= gcc -O3 -S foo.c =====================
gcc2_compiled.:
___gnu_compiled_c:
.text
	.align 4
	.global _mylist_insert
	.proc	020
_mylist_insert:
	!#PROLOGUE# 0
	!#PROLOGUE# 1
	ld [%o0],%g2		; fetch *p
	st %g2,[%o1+4]		; store it in memory
	ld [%o0],%g3		; <-------- fetch *p again, superfluous
	cmp %g3,0
	be L4
	add %o1,4,%g2
	st %g2,[%g3+8]
L4:
	st %o0,[%o1+8]
	retl
	st %o1,[%o0]
================================================================
Compilation log:
$ gcc -v -O3 -S -Wall foo.c
Reading specs from /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/lib/gcc-lib/sparc-sun-sunos4.1.3/2.7.2/specs
gcc version 2.7.2
 /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/lib/gcc-lib/sparc-sun-sunos4.1.3/2.7.2/cpp -lang-c -v -undef -D__GNUC__=2 -D__GNUC_MINOR__=7 -Dsparc -Dsun -Dunix -D__GCC_NEW_VARARGS__ -D__sparc__ -D__sun__ -D__unix__ -D__GCC_NEW_VARARGS__ -D__sparc -D__sun -D__unix -Asystem(unix) -Asystem(bsd) -Acpu(sparc) -Amachine(sparc) -D__OPTIMIZE__ -Wall foo.c /usr/tmp/cca11611.i
GNU CPP version 2.7.2 (sparc)
#include "..." search starts here:
#include <...> search starts here:
 /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/include
 /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/sparc-sun-sunos4.1.3/include
 /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/lib/gcc-lib/sparc-sun-sunos4.1.3/2.7.2/include
 /usr/include
End of search list.
 /nfs/ishare/lib/gnu/gcc/2.7.2/sunos/lib/gcc-lib/sparc-sun-sunos4.1.3/2.7.2/cc1 /usr/tmp/cca11611.i -quiet -dumpbase foo.c -O3 -Wall -version -o foo.s
GNU C version 2.7.2 (sparc) compiled by GNU C version 2.7.2.
$ 


Bruno Haible                                     email: <haible@ilog.fr>
Software Engineer                                phone: +33-1-49083585