In short, when passing a bitfield as parameter, each bit is converted to bytes
before being tested instead of a simple mask and test.
Is there a way to disable bit to bytes conversion by a compilation switch,
I have never seen that producing better code up to now?
$ cat tmp2.c
struct mouse_button_str {
unsigned char left : 1;
unsigned char right : 1;
unsigned char middle : 1;
} button;
char fct (struct mouse_button_str newbutton)
{
return (newbutton.left && newbutton.right && newbutton.middle);
}
$ ./toolchain/bin/gcc --version
gcc (GCC) 4.3.2 20080819 (prerelease)
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ ./toolchain/bin/gcc -Os tmp2.c -S -o tmp2.s
$ cat tmp2.s
.file "tmp2.c"
.text
.globl fct
.type fct, @function
fct:
pushl %ebp
movl %esp, %ebp
movb 8(%ebp), %al
movb %al, %cl
movb %al, %dl
shrb %cl
shrb $2, %dl
andl $1, %ecx
andl $1, %edx
testb $1, %al
je .L2
testb %cl, %cl
je .L2
movl %edx, %eax
andl $1, %eax
jmp .L3
.L2:
xorl %eax, %eax
.L3:
popl %ebp
ret
.size fct, .-fct
.comm button,1,1
.ident "GCC: (GNU) 4.3.2 20080819 (prerelease)"
.section .note.GNU-stack,"",@progbits
$ ./toolchain-4.3.1/bin/gcc --version
gcc (GCC) 4.3.1
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ ./toolchain-4.3.1/bin/gcc -Os tmp2.c -S -o tmp2.s
$ cat tmp2.s
.file "tmp2.c"
.text
.globl fct
.type fct, @function
fct:
pushl %ebp
movl %esp, %ebp
movb 8(%ebp), %al
popl %ebp
andl $7, %eax
cmpb $7, %al
sete %al
ret
.size fct, .-fct
.comm button,1,1
.ident "GCC: (GNU) 4.3.1"
.section .note.GNU-stack,"",@progbits
--
Summary: regression 4.3.1 -> 4.3.2-rc transformation bitfield to
individual bytes
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: etienne_lorrain at yahoo dot fr
GCC build triplet: i386-linux-fedora
GCC host triplet: i386-linux-fedora
GCC target triplet: i386-linux-fedora
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37248
