From mboxrd@z Thu Jan  1 00:00:00 1970
From: hjl@lucon.org (H.J. Lu)
To: acs@alumni.princeton.edu
Cc: jinbo21@soback.kornet.nm.kr, egcs@cygnus.com
Subject: Re: Different Test Results w/ -march=pentiumpro
Date: Thu, 07 May 1998 19:33:00 -0000
Message-id: <m0yXaE8-000268C@ocean.lucon.org>
References: <199805070316.XAA18129@spacely.icd.teradyne.com>
X-SW-Source: 1998-05/msg00260.html

> 
> On my UnixWare 2.1.2 (i386-pc-sysv4.2uw2.1.2) system with the 0502 snapshot,
> I get different test results when I add -march=pentiumpro to BOOT_CFLAGS.
> 
> In the first case I built with:
> BOOT_CFLAGS='-O3 -malign-jumps=4 -malign-loops=4 -malign-functions=4'
> 
> while in the second, I added -march=pentiumpro:
> BOOT_CFLAGS='-O3 -march=pentiumpro -malign-jumps=4 -malign-loops=4 -malign-functions=4'
> 
> 
> Here are the diffs in the summaries:
> 
> diff ~/tmp/egcs-19980502.results egcs-19980502.results-ppro
> 43a44,48
> > FAIL: gcc.c-torture/execute/loop-4.c execution,  -O2 
> > FAIL: gcc.c-torture/execute/loop-4.c execution,  -O2 -fomit-frame-pointer -finline-functions 
> > FAIL: gcc.c-torture/execute/loop-4.c execution,  -O2 -fomit-frame-pointer -finline-functions -funroll-loops 
> > FAIL: gcc.c-torture/execute/loop-4.c execution,  -O2 -fomit-frame-pointer -finline-functions -funroll-all-loops 
> > FAIL: gcc.c-torture/execute/loop-4.c execution,  -O2 -g 

There is a bug between final_scan_insn () and notice_update_cc () on
x86. final_scan_insn () may optimize out a test/compare instruction
when egcs thinks the condition code is already set up as desired.
The problem is notice_update_cc on x86 will update the condition code
for all kinds of instructions, including

	subl   %ecx,%eax

For

	subl   %ecx,%eax
	movl   %eax,0xffffffd4(%ebp)
	testl  %eax,%eax

when egcs sees "testl  %eax,%eax", it knows the condition code
is changed by "subl   %ecx,%eax" and "testl  %eax,%eax" is ignored.

Now we get a problem. Since "subl   %ecx,%eax" may cause overflow,
the condition code will be bogus in this case. Because of this,
check_dbra_loop () is miscompiled for PPro.

One way to fix it is to disable the condition code optimization. Or
we can add a bit to cc_status.flags, CC_UPDATED_BY_TEST_COMPARE.
We set the CC_UPDATED_BY_TEST_COMPARE bit in NOTICE_UPDATE_CC and
final_scan_insn () check the CC_UPDATED_BY_TEST_COMPARE bit before
optimizing out the condition code.

BTW, this bug may be cpu independent and may exist in egcs 1.0.x. We 
just happen to see egcs itself be miscompiled this time.


H.J.