From gcc-bugs  Fri Feb 18 18:45:31 2000
From: Leon Bottou <leonb () research ! att ! com>
Date: Fri, 18 Feb 2000 18:45:31 +0000
To: gcc-bugs
Subject: GCC-2.95 - bug in i386 pic code generation when using inline asm.
X-MARC-Message: https://marc.info/?l=gcc-bugs&m=95089957521804


leonb@tamil% uname -a
Linux tamil 2.2.5 #5 SMP Fri Sep 24 10:49:52 EDT 1999 i686 unknown

leonb@tamil% gcc -v
Reading specs from
/home/opt/libc6/egcs-2.95/lib/gcc-lib/i686-pc-linux-gnu/2.95/specs
gcc version 2.95 19990728 (release)

leonb@tamil% cat > bug.c <<EOF
  int x = 0; 
  int main() { asm("xor %%ebx,%%ebx" : : : "ebx"); return x; }
EOF
		-- Note that ebx is clobbered and declared as such.
		-- The asm statement should be a noop

leonb@tamil% gcc -fpic bug.c -o bug
leonb@tamil% bug
Segmentation fault (core dumped)		// WOW - It works without -fpic

leonb@tamil% gcc -fpic -S bug.c

leonb@tamil% cat bug.s
......
main:
        pushl %ebp
        movl %esp,%ebp
        pushl %ebx
        call .LPR0
        addl $_GLOBAL_OFFSET_TABLE_,%ebx
#APP
        xor %ebx,%ebx		// Ok
#NO_APP
        movl x@GOT(%ebx),%eax	// WOW: PIC uses EBX but forgot that I clobbered
it!
        movl (%eax),%edx
        movl %edx,%eax
        jmp .L2
        .p2align 4,,7
.L2:
        movl -4(%ebp),%ebx
        movl %ebp,%esp
        popl %ebp
        ret
.....