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* optimization/6004: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
@ 2002-03-19 9:36 Daniel.Diaz
0 siblings, 0 replies; 3+ messages in thread
From: Daniel.Diaz @ 2002-03-19 9:36 UTC (permalink / raw)
To: gcc-gnats
>Number: 6004
>Category: optimization
>Synopsis: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Tue Mar 19 09:36:01 PST 2002
>Closed-Date:
>Last-Modified:
>Originator: Daniel Diaz
>Release: gcc version 3.0.4 (Red Hat Linux 7.2 3.0.4-1)
>Organization:
>Environment:
RedHat Linux 7.2 (kernel 2.4.7-10) on a Pentium IV 1.7Ghz with 512Mb
>Description:
I use ebp as a global register. I inform gcc with -ffixed-ebp and also I compile with
-ffomit-frame-pointer (since else ebp is used for the frame).
Everything works well except for the main() function which always uses a frame and thus
refers its arguments from ebp (and not from esp). Since ebp is used by my global variable
the arguments are not well get by main().
Here is the file x.c (also as an attachment):
register int x asm("%ebp");
main(int argc)
{
foo(argc);
printf("Inside main() argc = %d\n", argc);
}
foo(int argc)
{
printf("Inside foo() argc = %d\n", argc);
x = 100;
}
Compite it with:
gcc -O2 -fomit-frame-pointer -ffixed-ebp x.c
at execution:
$ ./a.out
Inside foo() argc = 1
Segmentation fault
If we look at the assembly code (adding -S):
gcc -O2 -fomit-frame-pointer -ffixed-ebp x.c -S
we obtain:
.file "x.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "Inside main() argc = %d\n"
.text
.align 4
.globl main
.type main,@function
main:
pushl %ebp
movl %esp, %ebp
pushl %ebx
pushl %ecx
andl $-16, %esp
movl 8(%ebp), %ebx
subl $12, %esp
pushl %ebx
call foo
popl %eax
popl %edx
pushl %ebx
pushl $.LC0
call printf
movl -4(%ebp), %ebx
leave
ret
.Lfe1:
.size main,.Lfe1-main
.section .rodata.str1.1,"aMS",@progbits,1
.LC1:
.string "Inside foo() argc = %d\n"
.text
.align 4
.globl foo
.type foo,@function
foo:
subl $20, %esp
pushl 24(%esp)
pushl $.LC1
call printf
movl $100, %ebp
addl $28, %esp
ret
.Lfe2:
.size foo,.Lfe2-foo
.ident "GCC: (GNU) 3.0.4 (Red Hat Linux 7.2 3.0.4-1)"
we can see that the main() uses ebp to save esp creating a fram
and then (after the call to foo()) uses ebp to pass argc to printf().
But ebp has been set to 100 by foo() when initializing x to 100.
Obviously when main() camms printf(), argc is not well passed and a
segmentation violation occurs.
This is a serious problem for the GNU Prolog compiler which uses ebp
to store a global Prolog abstract machine register.
*** This is a new behavior in gcc 3.0.4 (and surely in 3.0.x). ***
*** This DID/DOES NOT occur in 2.96 20000731. ***
To summarize: gcc 3.0.4 ALWAYS uses a frame with ebp for amin()
even in the presence of -fomit-frame-pointer -ffixed-ebp.
A smaller example: let y.c the following file:
main()
{
}
foo()
{
}
compile it with:
gcc -O2 -fomit-frame-pointer -ffixed-ebp y.c -S
Here is y.s
.file "y.c"
.text
.align 4
.globl main
.type main,@function
main:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
andl $-16, %esp
leave
ret
.Lfe1:
.size main,.Lfe1-main
.align 4
.globl foo
.type foo,@function
foo:
ret
.Lfe2:
.size foo,.Lfe2-foo
.ident "GCC: (GNU) 3.0.4 (Red Hat Linux 7.2 3.0.4-1)"
So main() is NOT treated as foo() (or other functions) and uses frame+ebp whatever we declare !
>How-To-Repeat:
gcc -O2 -fomit-frame-pointer -ffixed-ebp x.c
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="x.c"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="x.c"
cmVnaXN0ZXIgaW50IHggYXNtKCIlZWJwIik7CgptYWluKGludCBhcmdjKQp7CiAgZm9vKGFyZ2Mp
OwogIHByaW50ZigiSW5zaWRlIG1haW4oKSBhcmdjID0gJWRcbiIsIGFyZ2MpOwp9Cgpmb28oaW50
IGFyZ2MpCnsKICBwcmludGYoIkluc2lkZSBmb28oKSBhcmdjID0gJWRcbiIsIGFyZ2MpOwogIHgg
PSAxMDA7Cn0KCg==
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: optimization/6004: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
@ 2002-03-20 1:57 jakub
0 siblings, 0 replies; 3+ messages in thread
From: jakub @ 2002-03-20 1:57 UTC (permalink / raw)
To: Daniel.Diaz, gcc-bugs, gcc-prs, nobody
Synopsis: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
State-Changed-From-To: open->analyzed
State-Changed-By: jakub
State-Changed-When: Wed Mar 20 01:57:57 2002
State-Changed-Why:
-fomit-frame-pointer documentation sais:
Don't keep the frame pointer in a register for functions that
don't need one, ...
which means the compiler makes no guarantees it will be
actually eliminated.
Particularly main if it aligns the stack pointer.
You might try
gcc -O2 -fomit-frame-pointer -ffixed-ebp -mpreferred-stack-boundary=2 x.c
for the .c file containing main.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6004
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: optimization/6004: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
@ 2002-04-03 2:36 rth
0 siblings, 0 replies; 3+ messages in thread
From: rth @ 2002-04-03 2:36 UTC (permalink / raw)
To: Daniel.Diaz, gcc-bugs, gcc-prs, nobody
Synopsis: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp
State-Changed-From-To: analyzed->closed
State-Changed-By: rth
State-Changed-When: Wed Apr 3 02:36:04 2002
State-Changed-Why:
Correct analysis. One cannot absolutely prevent the
compiler from using the hard frame pointer.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6004
^ permalink raw reply [flat|nested] 3+ messages in thread
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2002-03-19 9:36 optimization/6004: gcc 3.0.4 always compiles main() with frame (and ebp) even with -fomit-frame-pointer and -ffixed-ebp Daniel.Diaz
2002-03-20 1:57 jakub
2002-04-03 2:36 rth
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