8878 – [3.3 only] miscompilation with -O and SSE (original) (raw)

| gcc produce wrong assembly when using vector instructions through built-in functions and -O. I was benchmarking pure FPU vs. SSE, so the program is a loop that multiply and add 2 vector. Release: 3.2.1 Environment: System: Linux dreamland 2.4.20-rc3-xfs-acpi #9 Wed Nov 27 18:01:32 CET 2002 i686 unknown Architecture: i686 host: i686-pc-linux-gnu build: i686-pc-linux-gnu target: i686-pc-linux-gnu configured with: ../gcc/configure --prefix=/usr --enable-shared --with-slibdir=/lib --with-gnu-as --with-gnu-ld --enable-threads --enable-languages=c,c++ How-To-Repeat: #include <stdio.h> int main(void) { typedef int v4sf __attribute__ ((mode(V4SF))); v4sf a = {2.0, 2.0, 2.0, 2.0}; v4sf b = {1.0, 2.0, 3.0, 4.0}; v4sf c = {0.0, 0.0, 0.0, 0.0}; v4sf d; int i; for(i = 0; i < 1000000; i++) { d = __builtin_ia32_mulps(a, b); c = __builtin_ia32_addps(c, d); } for (i = 0; i < 4; i++) printf("%f ", *(((float *)(&c)) + i)); printf("\n"); return 0; } Compiled with: gcc -Wall -ofloat-sse -march=athlon-xp float-sse.c it gives me the following (correct) output: kronos:~/c$ float-sse 2000000.000000 4000000.000000 6000000.000000 8000000.000000 Compiled with: gcc -O -Wall -ofloat-sse -march=athlon-xp float-sse.c it gives a wrong output: kronos:~/c$ float-sse 8000000.000000 0.000000 0.000000 0.000000 Assembly output available if needed. Comment 1 kronoz 2002-12-09 09:56:04 UTC Fix: Don't know. Comment 2 Volker Reichelt 2002-12-11 06:08:13 UTC State-Changed-From-To: open->analyzed State-Changed-Why: Confirmed. Here's a cleaned-up testcase (which does not suffer from aliasing problems as the original one, where &c is casted): ----------------------------snip here------------------------- #include <stdio.h> typedef int v4sf __attribute__((mode(V4SF))); int main(void) { v4sf v = {1.0, 2.0, 3.0, 4.0}; union { v4sf v; float f[4]; } u; u.v = __builtin_ia32_mulps(v,v); printf("%f %f %f %f\n", u.f[0], u.f[1], u.f[2], u.f[3]); return 0; } ----------------------------snip here------------------------- Compiling this with "gcc -O -msse" on a i686-pc-linux-gnu machine results in an executable that prints 16.000000 0.000000 0.000000 0.000000 instead of 1.000000 4.000000 9.000000 16.000000 as expeceted. Replacing "mulps" by "addps" will generate equally wrong results. Comment 3 janis187 2003-04-02 11:44:27 UTC From: Janis Johnson <janis187@us.ibm.com> To: Volker Reichelt <reichelt@igpm.rwth-aachen.de> Cc: janis187@us.ibm.com, gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org, kronoz@tiscali.it Subject: Re: optimization/8878: miscompilation with -O and SSE Date: Wed, 2 Apr 2003 11:44:27 -0800 On Wed, Apr 02, 2003 at 09:17:49PM +0200, Volker Reichelt wrote: > Hi Janis, > > PR 8878 got fixed on mainline in the last couple of days (somewhere > between 2003-03-29 and 2003-04-02). > Since the bug is in the category "wrong-code" it would be nice > if the patch could be backported to 3.3 (and maybe even 3.2) to > prevent silent miscompilations. > > Could you please identify the patch that fixed the problem? > Maybe the following testcase is more convenient for the hunt, > since you can check the return value instead of the output: > > ------------------------snip here---------------------------- > typedef int v4sf __attribute__((mode(V4SF))); > > int main(void) > { > v4sf v = {1.0, 2.0, 3.0, 4.0}; > union { v4sf v; float f[4]; } u; > > u.v = __builtin_ia32_mulps(v,v); > > return u.f[0]; > } > ------------------------snip here---------------------------- > > Just compile with "gcc -march=i686 -msse -O". I don't know much about x86 architectures and what's supposed to work where, but on my Pentium III this gets "Illegal instruction" when run, using a mainline compiler from sources updated yesterday. Janis Comment 4 Volker Reichelt 2003-04-02 21:17:49 UTC From: Volker Reichelt <reichelt@igpm.rwth-aachen.de> To: janis187@us.ibm.com Cc: gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org, kronoz@tiscali.it Subject: Re: optimization/8878: miscompilation with -O and SSE Date: Wed, 02 Apr 2003 21:17:49 +0200 (CEST) Hi Janis, PR 8878 got fixed on mainline in the last couple of days (somewhere between 2003-03-29 and 2003-04-02). Since the bug is in the category "wrong-code" it would be nice if the patch could be backported to 3.3 (and maybe even 3.2) to prevent silent miscompilations. Could you please identify the patch that fixed the problem? Maybe the following testcase is more convenient for the hunt, since you can check the return value instead of the output: ------------------------snip here---------------------------- typedef int v4sf __attribute__((mode(V4SF))); int main(void) { v4sf v = {1.0, 2.0, 3.0, 4.0}; union { v4sf v; float f[4]; } u; u.v = __builtin_ia32_mulps(v,v); return u.f[0]; } ------------------------snip here---------------------------- Just compile with "gcc -march=i686 -msse -O". Thanks, Volker http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8878 Comment 6 Volker Reichelt 2003-05-02 16:34:14 UTC Responsible-Changed-From-To: unassigned->aldyh Responsible-Changed-Why: This problem is probably in your domain. Can you fix that on the 3.3 branch or is this pure 3.4 stuff (where it is fixed already)? BTW, I now get an ICE on the 3.3 branch (as of 20030502): PR8878.c: In function `main': PR8878.c🔞 internal compiler error: in subreg_hard_regno, at emit-rtl.c:931 Please submit a full bug report, [etc.] Comment 7 Dara Hazeghi 2003-07-06 02:00:47 UTC Note that the ICE on 3.3 branch is now gone, but the code is still wrong. Comment 8 Volker Reichelt 2003-07-09 13:41:20 UTC I still get the ICE on the branch (20030709). Maybe that's because I'm using --enable-checking. | | | 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