[Bug c/13260] New: Incorrect optimisation of loop termination condition

[Bug c/13260] New: Incorrect optimisation of loop termination condition

[stuart dot menefy at st dot com]

This problem was first seen when building the Linux kernel, in the function
root_nfs_parse_addr(). I've extracted this into a small stand alone
program which demonstrates the problem.

When built with an older gcc the results are as expected, however this fails
on more recent versions of gcc. Turning the optimisation down to -O1 also
allows the program to work correctly. Also uncommenting the printf in the
middle of the code makes it work correctly.

Of the gcc versions I've tried, versions 3.0.3 and the 3.2.1
(prerelease) which SuperH ship work OK. However version 3.3.1, 3.3.2
and the latest 3.4 snapshot (20031126) all generate incorrect code.

The problem centers around a new optimisation which gcc has spotted, but
doesn't appear to get quite right.

In gcc 3.2.1 the code generated looks like:
	cmp/gt	r3,r7
	movt	r2
	tst	r2,r2
	movt	r1
	cmp/pl	r2
	add	r1,r8
	bf/s	.L16
	mov	r8,r3

In 3.3.2 it has spotted that after the 'tst' the T flag already
indicates whether to take the branch or not, and so the extra register
and comparison are redundant. However, to do this the branch has to be
reversed into a bt/s, and this is not being done:
	cmp/gt	r3,r7
	movt	r1
	tst	r1,r1
	movt	r1
	add	r1,r8
	bf/s	.L16
	mov	r8,r3

The code generated by 3.4 (20031126) is identical.

--------------
Compilation diagnostics:
% /var/tmp/gcc-3.4-20031126/objdir/gcc/xgcc
-B/var/tmp/gcc-3.4-20031126/objdir/gcc/ -O2 -S ipconfig.c -nostdinc -I
/opt/STM/ST40R2.1.3/sh-superh-elf/include/ -v -save-temps
Reading specs from /var/tmp/gcc-3.4-20031126/objdir/gcc/specs
Configured with: ../configure --host=i686-pc-linux-gnu --target=sh4-linux --pref
ix=/opt/STM/ST40Linux-1.0/devkit/sh4 --exec-prefix=/opt/STM/ST40Linux-1.0/devkit
/sh4 --bindir=/opt/STM/ST40Linux-1.0/devkit/sh4/bin --sbindir=/opt/STM/ST40Linux
-1.0/devkit/sh4/sbin --sysconfdir=/opt/STM/ST40Linux-1.0/devkit/sh4/etc --datadi
r=/opt/STM/ST40Linux-1.0/devkit/sh4/share --includedir=/opt/STM/ST40Linux-1.0/de
vkit/sh4/include --libdir=/opt/STM/ST40Linux-1.0/devkit/sh4/lib --libexecdir=/op
t/STM/ST40Linux-1.0/devkit/sh4/libexec --localstatedir=/opt/STM/ST40Linux-1.0/de
vkit/sh4/var --sharedstatedir=/opt/STM/ST40Linux-1.0/devkit/sh4/share --mandir=/
opt/STM/ST40Linux-1.0/devkit/sh4/man --infodir=/opt/STM/ST40Linux-1.0/devkit/sh4
/info --program-transform-name=s,^,sh4-linux-, --enable-shared --enable-language
s=c,c++ --enable-threads=posix --enable-c99 --enable-long-long --with-system-zli
b --disable-multilib --enable-symvers=gnu --enable-__cxa_atexit --with-gxx-inclu
de-dir=${prefix}/target//usr/include/g++-
Thread model: posix
gcc version 3.4 20031126 (experimental)
 /var/tmp/gcc-3.4-20031126/objdir/gcc/cc1 -E -quiet -nostdinc -v -I /opt/STM/ST4
0R2.1.3/sh-superh-elf/include/ -iprefix /var/tmp/gcc-3.4-20031126/objdir/gcc/../
lib/gcc/sh4-linux/3.4/ -isystem /var/tmp/gcc-3.4-20031126/objdir/gcc/include ipc
onfig.c -O2 -o ipconfig.i
#include "..." search starts here:
#include <...> search starts here:
 /opt/STM/ST40R2.1.3/sh-superh-elf/include/
 /var/tmp/gcc-3.4-20031126/objdir/gcc/include
End of search list.
 /var/tmp/gcc-3.4-20031126/objdir/gcc/cc1 -fpreprocessed ipconfig.i -quiet -dump
base ipconfig.c -auxbase ipconfig -O2 -version -o ipconfig.s
GNU C version 3.4 20031126 (experimental) (sh4-linux)
        compiled by GNU C version 3.2.2 20030222 (Red Hat Linux 3.2.2-5).
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096

-- 
           Summary: Incorrect optimisation of loop termination condition
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: stuart dot menefy at st dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: sh4-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug c/13260] Incorrect optimisation of loop termination condition

[paolo at gcc dot gnu dot org]

------- Additional Comments From paolo at gcc dot gnu dot org  2003-12-01 15:51 -------
Hi.Thanks for your report, but eventually you forgot to attach the snippet!

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |critical
          Component|c                           |optimization
           Keywords|                            |wrong-code
            Summary|Incorrect optimisation of   |[3.4 Regression] Incorrect
                   |loop termination condition  |optimisation of loop
                   |                            |termination condition
   Target Milestone|---                         |3.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-01 16:04 -------
Marking as invalid to mark it as ...

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-01 16:04 -------
unconfirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2003-12-04 20:10 -------
Subject: Bug 13260

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	amylaar@gcc.gnu.org	2003-12-04 20:10:29

Modified files:
	gcc            : ChangeLog 
	gcc/config/sh  : sh-protos.h sh.c sh.h sh.md 

Log message:
	PR optimization/13260
	* sh-protos.h (sh_expand_t_scc): Declare.
	* sh.h (PREDICATE_CODES): Add cmpsi_operand.
	* sh.c (cmpsi_operand, sh_expand_t_scc): New functions.
	* sh.md (cmpsi): Use cmpsi_operand.  If T_REG is compared to
	something that is not a CONST_INT, copy it into a pseudo register.
	(subc): Fix description of new T value.
	(slt, sgt, sge, sgtu): Don't clobber T after rtl generation is over.
	(sltu, sleu, sgeu): Likewise.
	(seq, sne): Likewise. Use sh_expand_t_scc.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.1915&r2=2.1916
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/config/sh/sh-protos.h.diff?cvsroot=gcc&r1=1.50&r2=1.51
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/config/sh/sh.c.diff?cvsroot=gcc&r1=1.239&r2=1.240
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/config/sh/sh.h.diff?cvsroot=gcc&r1=1.227&r2=1.228
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/config/sh/sh.md.diff?cvsroot=gcc&r1=1.162&r2=1.163



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[amylaar at gcc dot gnu dot org]

------- Additional Comments From amylaar at gcc dot gnu dot org  2003-12-04 20:35 -------
Created an attachment (id=5275)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=5275&action=view)
patch which has been applied to mainline (3.4 experimental)


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |amylaar at gcc dot gnu dot
                   |dot org                     |org
             Status|UNCONFIRMED                 |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2003-12-04 21:02 -------
Subject: Bug 13260

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	amylaar@gcc.gnu.org	2003-12-04 21:02:34

Modified files:
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/gcc.c-torture/execute: 20031204-1.c 

Log message:
	2003-12-04  Stuart Menefy <stuart.menefy@st.com>
	J"orn Rennecke <joern.rennecke@superh.com>
	
	PR optimization/13260
	* gcc.c-torture/execute/20031204-1.c: New test.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.3226&r2=1.3227
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.c-torture/execute/20031204-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[amylaar at gcc dot gnu dot org]

------- Additional Comments From amylaar at gcc dot gnu dot org  2003-12-04 21:07 -------
Fixed with patch

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260

[Bug optimization/13260] [3.4 Regression] Incorrect optimisation of loop termination condition

[dank at kegel dot com]

------- Additional Comments From dank at kegel dot com  2004-02-09 16:03 -------
A backport of this patch to gcc-3.3.3 was written by SUGIOKA Toshinobu
and is (mistakenly) archived as
http://www.m17n.org/linux-sh/ml/linux-sh/2003-02/msg00014.html

It causes no regressions on sh4 in gcc-3.3.3-20040131's C, C++, 
and libstdc++ testsuites, nor in the part of the glibc-2.3.2 
testsuite that I usually run.

I was surprised and pleased to see that it also fixed my testcase for PR11587!


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13260