On Aug 7, 2009, at 6:41 PM, Douglas Gregor wrote:
> URL: http://llvm.org/viewvc/llvm-project?rev=78450&view=rev
> Log:
> Introduce reference counting for statements and expressions, using it
> to allow sharing of nodes. Simplifies some aspects of template
> instantiation, and fixes both PR3444 and <rdar://problem/6757457>.
Yay for progress :)
> + /// \brief The statement class.
> + const unsigned sClass : 8;
> +
> + /// \brief The reference count for this statement.
> + unsigned RefCount : 24;
Random thought, do you ever expect to need more than 65536 ref
counts? If not, shrinking the refcount to be an unsigned short would
lead to slightly more efficient generated code for some operations. If
you compare this:
struct x {
#ifdef BITFIELD
unsigned int x : 8;
unsigned int y : 24;
#else
unsigned char x;
unsigned short y;
#endif
};
void foo(struct x *P) {
if (--P->y == 0)
bar();
}
compiled with gcc 4.2, I get:
NONBITFIELD: gcc t.c -S -o - -O3 -m32 -fomit-frame-pointer -fno-
optimize-sibling-calls
_foo:
subl $12, %esp
movl 16(%esp), %eax
movzwl 2(%eax), %edx
decl %edx
movw %dx, 2(%eax)
testw %dx, %dx
jne L4
call _bar
L4:
addl $12, %esp
ret
BITFIELD: gcc t.c -S -o - -O3 -m32 -fomit-frame-pointer -fno-optimize-
sibling-calls -DBITFIELD
_foo:
pushl %esi
subl $8, %esp
movl 16(%esp), %esi
movl (%esi), %eax
movl %eax, %edx
shrl $8, %edx
decl %edx
andl $16777215, %edx
movl %edx, %ecx
sall $8, %ecx
andl $255, %eax
orl %ecx, %eax
movl %eax, (%esi)
testl %edx, %edx
jne L4
call _bar
L4:
addl $8, %esp
popl %esi
ret
Clang does a better job than GCC, but still suffers:
NONBITFIELD:
_foo:
subl $12, %esp
movl 16(%esp), %eax
movw 2(%eax), %cx
decw %cx
movw %cx, 2(%eax)
testw %cx, %cx
jne LBB1_2
call _bar
LBB1_2:
addl $12, %esp
ret
BITFIELD: clang t.c -S -o - -O3 -m32 -fomit-frame-pointer -DBITFIELD
_foo:
pushl %esi
subl $8, %esp
movl 16(%esp), %eax
movl (%eax), %ecx
movzbl %cl, %edx
shrl $8, %ecx
addl $16777215, %ecx
movl %ecx, %esi
shll $8, %esi
orl %edx, %esi
movl %esi, (%eax)
testl $16777215, %ecx
jne LBB1_2
call _bar
LBB1_2:
addl $8, %esp
popl %esi
ret
Incidentally, llvm-gcc produces slightly code for the bitfield case
than the non-bitfield case, and wipes the floor of either clang or gcc:
NONBITFIELD:
_foo:
subl $12, %esp
movl 16(%esp), %eax
movw 2(%eax), %cx
movw %cx, %dx
decw %dx
movw %dx, 2(%eax)
cmpw $1, %cx
jne LBB1_2
call _bar
LBB1_2:
addl $12, %esp
ret
BITFIELD:
_foo:
subl $12, %esp
movl 16(%esp), %eax
movl $4294967040, %ecx
addl (%eax), %ecx
movl %ecx, (%eax)
cmpl $255, %ecx
ja LBB1_2
call _bar
LBB1_2:
addl $12, %esp
ret
-Chris
_______________________________________________
cfe-commits mailing list
[email protected]
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
