c/4700: GCC not honoring asm clobbered regesters for -fPIC on i386

[ddaney@avtrex.com]

>Number:         4700
>Category:       c
>Synopsis:       GCC not honoring asm clobbered regesters for -fPIC on i386
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 26 10:56:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     David Daney
>Release:        gcc 3.0.1
>Organization:
>Environment:
Linux dl 2.4.13-1 #1 Wed Oct 24 14:35:01 PDT 2001 i686 unknown
GNU assembler 2.11.2
GNU ld 2.11.2
>Description:
The test case for the problem is an inline function that
contains only asm code.

When compiled with -fPIC it seems that gcc uses the register ebx to point to the static data area.  However the asm statement says that the asm code clobbers ebx.  Gcc should save ebx and then restore its value after the asm block, but it does not.  This leads to SEGV when the clobbered ebx is used to access static data.
>How-To-Repeat:
break the included file into its three parts 
clobber.c clobbertest.c and Makefile

Then build with make all.

The executable clobberbad will SEGV when run.
The executable clobbergood will complete with out problems.

clobberbad links to the asm code compiled with -fPIC and clobbergood links to the same code but without -fPIC.
>Fix:
explicitly save and restore all clobbered registers in asm statements and then do not use the asm clobbers clause.
>Release-Note:
>Audit-Trail:
>Unformatted:
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