Date: Tue, 7 Mar 2000 10:24:54 +0100
Message-Id: <200003070924.KAA14711@virtutech.se>
X-Authentication-Warning: skynet.hq.vtech: mch set sender to mch AT virtutech DOT se using -f
From: Magnus Christensson <mch AT virtutech DOT se>
To: pgcc AT delorie DOT com
Subject: -fcompare-elim bug
Reply-To: pgcc AT delorie DOT com


Pgcc 2.95.2 generates broken code with -fcompare-elim enabled.

I'm using pgcc version pgcc-2.95.2 19991024 (release) on a
GNU/Linux(x86) system.

Code from 186.crafty (a specint2000 benchmark):
/*
    enpassant pawns are on either file adjacent to the current file, and
    on the same rank.                                          
*/
  for (i=0;i<64;i++) mask_eptest[i]=0;
  for (i=25;i<31;i++) mask_eptest[i]=Or(set_mask[i-1],set_mask[i+1]);
  for (i=33;i<39;i++) mask_eptest[i]=Or(set_mask[i-1],set_mask[i+1]);
  mask_eptest[A4]=set_mask[B4];
  mask_eptest[H4]=set_mask[G4];
  mask_eptest[A5]=set_mask[B5];
  mask_eptest[H5]=set_mask[G5];

For the second for loop pgcc generates:
	;; mask_eptest: 0x80ae5c0
	;; set_mask: 0x81521c0
	;; Or(a,b) = (a) | (b)
	;; -O3 -g -mcpu=pentiumpro -march=pentiumpro
0x805f695 <InitializePawnMasks+1029>:	mov    $0xfffffffa,%edi
0x805f69a <InitializePawnMasks+1034>:	mov    $0xd0,%ecx
0x805f69f <InitializePawnMasks+1039>:	mov    $0xc0,%edx
0x805f6a4 <InitializePawnMasks+1044>:	mov    $0xc8,%eax
0x805f6a9 <InitializePawnMasks+1049>:	lea    0x0(%esi,1),%esi
0x805f6b0 <InitializePawnMasks+1056>:	mov    0x81521c0(%edx),%ebx
0x805f6b6 <InitializePawnMasks+1062>:	mov    0x81521c4(%edx),%esi
0x805f6bc <InitializePawnMasks+1068>:	or     0x81521c0(%ecx),%ebx
0x805f6c2 <InitializePawnMasks+1074>:	or     0x81521c4(%ecx),%esi
0x805f6c8 <InitializePawnMasks+1080>:	mov    %ebx,0x80ae5c0(%eax)
0x805f6ce <InitializePawnMasks+1086>:	mov    %esi,0x80ae5c4(%eax)
0x805f6d4 <InitializePawnMasks+1092>:	add    $0x8,%ecx
0x805f6d7 <InitializePawnMasks+1095>:	add    $0x8,%edx
0x805f6da <InitializePawnMasks+1098>:	add    $0x100,%eax
0x805f6df <InitializePawnMasks+1103>:	add    $0x20,%edi
0x805f6e2 <InitializePawnMasks+1106>:	
    jne    0x805f6b0 <InitializePawnMasks+1056>

The last add instruction will never generate an equal (zero) result,
so the loop will go on forever.

Code without -fcompare-elim:
	;; mask_eptest: 0x80ae580
	;; set_mask: 0x8152180
	;; -O3 -fno-compare-elim -g -mcpu=pentiumpro -march=pentiumpro
0x805f625 <InitializePawnMasks+1029>:	mov    $0x19,%edi
0x805f62a <InitializePawnMasks+1034>:	mov    $0x8152180,%ecx
0x805f62f <InitializePawnMasks+1039>:	mov    $0xd0,%edx
0x805f634 <InitializePawnMasks+1044>:	mov    $0xc0,%eax
0x805f639 <InitializePawnMasks+1049>:	lea    0x0(%esi,1),%esi
0x805f640 <InitializePawnMasks+1056>:	mov    (%eax,%ecx,1),%ebx
0x805f643 <InitializePawnMasks+1059>:	mov    0x4(%eax,%ecx,1),%esi
0x805f647 <InitializePawnMasks+1063>:	or     (%edx,%ecx,1),%ebx
0x805f64a <InitializePawnMasks+1066>:	or     0x4(%edx,%ecx,1),%esi
0x805f64e <InitializePawnMasks+1070>:	mov    %ebx,0x80ae580(,%edi,8)
0x805f655 <InitializePawnMasks+1077>:	mov    %esi,0x80ae584(,%edi,8)
0x805f65c <InitializePawnMasks+1084>:	add    $0x8,%edx
0x805f65f <InitializePawnMasks+1087>:	add    $0x8,%eax
0x805f662 <InitializePawnMasks+1090>:	inc    %edi
0x805f663 <InitializePawnMasks+1091>:	cmp    $0x1e,%edi
0x805f666 <InitializePawnMasks+1094>:	
    jle    0x805f640 <InitializePawnMasks+1056>

I'm not a member of any pgcc mailing lists, so please cc all
follow-ups to mch AT virtutech DOT se.

/Magnus