--- Begin Message ---
- To: submit@bugs.debian.org
- Subject: Non-optimal code generated by gcc-3.0
- From: Petr Vandrovec <vandrove@vc.cvut.cz>
- Date: Mon, 30 Jul 2001 19:27:15 +0200
- Message-id: <20010730192715.A5420@vana.vc.cvut.cz>
Package: gcc-3.0
Version: 3.0.1-0pre010727
Severity: wishlist
Hi,
this simple code snippet generates a bit suboptimal code
when compiled with gcc-3.0. For some unknown reason
simple '--b->cnt' is correctly assembled as 'decl (somememory)',
but when this expression is tested for zero (which is available
on i386 in Z flag after simple decrement), it loads value
into register, decrements, then even again tests for zero, and
finally stores it back to memory ... This is visible in both
freeblock and freeblockC.
Second problem is that when decremented field in structure
is not integer (for example char) and function is chained into
another one with same arguments, some of arguments are
for unknown reason written back to stack, although their
value did not change (I even tried to mark argument
as 'struct blockChar* const b', but no change on generated
code). This is visible in freeblockC only.
These are all acceptable because of gcc-2.95.x generated
even worse code.
But what's not acceptable happens in readAsyncCallback.
For some strange reason it allocates 16bytes of stack, for
unknown reason (note that it was compiled with
-mpreferred-stack-boundary=2, so it is not any stack aligning).
It is smallest example of problem I'm suffering of - I have
function which reserves 364 bytes on stack with gcc-3.0, but
only 120 bytes when compiled with gcc-2.95, and code generated by
gcc-3.0 does not even touch couple of on-stack variables
which were optimized, but then somehow left in stack frame.
Thanks,
Petr Vandrovec
vandrove@vc.cvut.cz
struct blockInt {
unsigned int cnt;
};
void __freeblock(struct blockInt*);
void freeblock(struct blockInt* b) {
if (--b->cnt) __freeblock(b);
}
void simpleDec(struct blockInt* b) {
--b->cnt;
}
struct blockChar {
unsigned char cnt;
void* address;
};
void __freeblockC(struct blockChar*);
void freeblockC(struct blockChar* b) {
if (--b->cnt) __freeblockC(b);
}
void simpleDecC(struct blockChar* b) {
--b->cnt;
}
struct semaphore {
void* queue;
int value;
};
struct LB_async {
struct semaphore* sema;
};
void LBL_sSignal(struct semaphore*);
static inline void __LBL_sSignal(struct semaphore* sem) {
if (!--sem->value && sem->queue) {
LBL_sSignal(sem);
}
}
void readAsyncCallback(struct LB_async* req) {
__LBL_sSignal(req->sema);
}
---------- generated asm ----------
.file "x.c"
# GNU C version 3.0.1 20010728 (Debian prerelease) (i386-linux)
# compiled by GNU C version 3.0.1 20010728 (Debian prerelease).
# options passed: -lang-c -D__GNUC__=3 -D__GNUC_MINOR__=0
# -D__GNUC_PATCHLEVEL__=1 -D__ELF__ -Dunix -Dlinux -D__ELF__ -D__unix__
# -D__linux__ -D__unix -D__linux -Asystem=posix -D__OPTIMIZE__
# -D__STDC_HOSTED__=1 -W -Wall -Acpu=i386 -Amachine=i386 -Di386 -D__i386
# -D__i386__ -D__tune_i386__ -mpreferred-stack-boundary=2 -O2 -W -Wall
# -fomit-frame-pointer -fverbose-asm
# options enabled: -fdefer-pop -fomit-frame-pointer
# -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks
# -fexpensive-optimizations -fthread-jumps -fstrength-reduce -fpeephole
# -fforce-mem -ffunction-cse -finline -fkeep-static-consts -fcaller-saves
# -fpcc-struct-return -fgcse -frerun-cse-after-loop -frerun-loop-opt
# -fdelete-null-pointer-checks -fschedule-insns2 -fsched-interblock
# -fsched-spec -fbranch-count-reg -freorder-blocks -fcommon -fverbose-asm
# -fgnu-linker -fregmove -foptimize-register-move -fargument-alias
# -fstrict-aliasing -fident -fpeephole2 -fguess-branch-probability
# -fmath-errno -m80387 -mhard-float -mno-soft-float -mieee-fp
# -mfp-ret-in-387 -mpreferred-stack-boundary=2
.text
.align 4
.globl freeblock
.type freeblock,@function
freeblock:
movl 4(%esp), %edx
movl (%edx), %eax
decl %eax
testl %eax, %eax
movl %eax, (%edx)
je .L2
jmp __freeblock
.p2align 2
.L2:
ret
.Lfe1:
.size freeblock,.Lfe1-freeblock
.align 4
.globl simpleDec
.type simpleDec,@function
simpleDec:
movl 4(%esp), %eax
decl (%eax)
ret
.Lfe2:
.size simpleDec,.Lfe2-simpleDec
.align 4
.globl freeblockC
.type freeblockC,@function
freeblockC:
movl 4(%esp), %edx
movb (%edx), %al
decl %eax
testb %al, %al
movb %al, (%edx)
je .L5
movl %edx, 4(%esp)
jmp __freeblockC
.p2align 2
.L5:
ret
.Lfe3:
.size freeblockC,.Lfe3-freeblockC
.align 4
.globl simpleDecC
.type simpleDecC,@function
simpleDecC:
movl 4(%esp), %eax
decb (%eax)
ret
.Lfe4:
.size simpleDecC,.Lfe4-simpleDecC
.align 4
.globl readAsyncCallback
.type readAsyncCallback,@function
readAsyncCallback:
subl $16, %esp
movl 20(%esp), %eax
movl (%eax), %edx
movl 4(%edx), %eax
decl %eax
testl %eax, %eax
movl %eax, 4(%edx)
je .L11
.L10:
addl $16, %esp
ret
.p2align 2
.L11:
movl (%edx), %ecx
testl %ecx, %ecx
je .L10
pushl %edx
call LBL_sSignal
popl %eax
jmp .L10
.Lfe5:
.size readAsyncCallback,.Lfe5-readAsyncCallback
.ident "GCC: (GNU) 3.0.1 20010728 (Debian prerelease)"
--- End Message ---