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c++/7443: gcc (mips) 3.0.3 incorrectly optimizes arithmetic containing 0x80000000
- From: ddolson at sandvine dot com
- To: gcc-gnats at gcc dot gnu dot org
- Cc: kfaiczak at sandvine dot com
- Date: 30 Jul 2002 14:30:48 -0000
- Subject: c++/7443: gcc (mips) 3.0.3 incorrectly optimizes arithmetic containing 0x80000000
- Reply-to: ddolson at sandvine dot com
>Number: 7443
>Category: c++
>Synopsis: gcc (mips) 3.0.3 incorrectly optimizes arithmetic containing 0x80000000
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Tue Jul 30 07:36:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: David Dolson
>Release: 3.0.3
>Organization:
>Environment:
Compiler running on cygwin on win2k
>Description:
There are several cases in which incorrect code is generated when the constant 0x80000000u is used in source code. Two such cases are included below.
Very simple code to reproduce: 48 lines of source and 62 lines of disassembly to demonstrate several cases.
The entire source code required is in the following bug.cpp
The objdump output is in the following bug.dmp
The defects are found at addresses 0x08, 0x30 and 0x58 of the disassembly: the function argument a0 is compared to zero without involving 0xf0000000 in any way.
Correct behaviour is shown in good_fn1(), in which a subu instruction is issued at address 0x88.
I could not work-around the issue with any variations in casting.
--------- Compiler config
Here is the first part of the -v output (wrapped at 80 cols):
Reading specs from ../../../Tornado/host/x86-win32/bin/../lib/gcc-lib/mips-wrs-v
xworks/3.0.3/specs
Configured with: ../gcc/configure --target=mips-wrs-vxworks --enable-languages=c
,c++
Thread model: vxworks
gcc version 3.0.3
../../../Tornado/host/x86-win32/bin/../lib/gcc-lib/mips-wrs-vxworks/3.0.3/cc1pl
us.exe -v -iprefix ../../../Tornado/host/x86-win32/bin/../lib/gcc-lib/mips-wrs-v
xworks/3.0.3/ -M -MF bug.d -MP -MQ bug.o -D__GNUC__=3 -D__GNUC_MINOR__=0 -D__GNU
C_PATCHLEVEL__=3 -D__mips__ -D__MIPSEB__ -D__R3000__ -D__mips__ -D_MIPSEB -D_R30
00 -D__mips -D__MIPSEB -D__R3000 -D__mips -D__OPTIMIZE__ -D__STDC_HOSTED__=1 -Wa
ll -D__LANGUAGE_C -D_LANGUAGE_C -D__SIZE_TYPE__=unsigned int -D__PTRDIFF_TYPE__=
int -U__mips -D__mips=3 -D__mips64 -D__mipsfp64 -UMIPSEB -U_MIPSEB -U__MIPSEB -U
__MIPSEB__ -D_MIPSEL -D__MIPSEL -D__MIPSEL__ bug.cpp -D__GNUG__=3 -D__GXX_DEPREC
ATED -D__EXCEPTIONS -D__GXX_ABI_VERSION=100 -D__STRICT_ANSI__ -trigraphs -$ -mfp
64 -mgp64 -G 0 -mel -quiet -dumpbase bug.cpp -mcpu=r4000 -mips3 -ansi -O1 -Wall
-ansi -version -o C:/Programs/cygwin/tmp/ccvgdZnl.s
GNU CPP version 3.0.3 (cpplib) [AL 1.1, MM 40] BSD Mips
GNU C++ version 3.0.3 (mips-wrs-vxworks)
compiled by GNU C version 2.95.3-5 (cygwin special).
------- bug.cpp starts here
void another_function();
// This function should return true in the following cases:
// 0 <= u <= 0x70000000
// 0xf0000000 < u <= 0xffffffff
bool bad_fn1(unsigned u)
{
if( 0xf0000000u - u >= 0x80000000u )
{
another_function();
return true;
}
return false;
}
// This function should return true in the following cases:
// 0 <= u <= 0x70000000
// 0xf0000000 < u <= 0xffffffff
bool bad_fn2(unsigned u)
{
if( (0xf0000000u - u) & 0x80000000u )
{
another_function();
return true;
}
return false;
}
// Should be same as bad_fn2, rephrased using signed arithmetic.
bool bad_fn3( unsigned u )
{
if( (signed int)( 0xf0000000u - u ) < 0 )
{
another_function();
return true;
}
return false;
}
bool good_fn1(unsigned u)
{
static unsigned u1 = 0xf0000000u;
if( u1 - u >= 0x80000000u )
{
another_function();
return true;
}
return false;
}
------- bug.dmp starts here
bug.o: file format elf32-littlemips
Disassembly of section .text:
0000000000000000 <bad_fn1(unsigned)>:
0: 27bdffd8 addiu sp,sp,-40
4: ffbf0020 sd ra,32(sp)
8: 04800004 bltz a0,1c <bad_fn1(unsigned)+0x1c>
c: 0000102d move v0,zero
10: 0c000000 jal 0 <bad_fn1(unsigned)>
10: R_MIPS_26 another_function()
14: 00000000 nop
18: 24020001 li v0,1
1c: dfbf0020 ld ra,32(sp)
20: 03e00008 jr ra
24: 27bd0028 addiu sp,sp,40
0000000000000028 <bad_fn2(unsigned)>:
28: 27bdffd8 addiu sp,sp,-40
2c: ffbf0020 sd ra,32(sp)
30: 04800004 bltz a0,44 <bad_fn2(unsigned)+0x1c>
34: 0000102d move v0,zero
38: 0c000000 jal 0 <bad_fn1(unsigned)>
38: R_MIPS_26 another_function()
3c: 00000000 nop
40: 24020001 li v0,1
44: dfbf0020 ld ra,32(sp)
48: 03e00008 jr ra
4c: 27bd0028 addiu sp,sp,40
0000000000000050 <bad_fn3(unsigned)>:
50: 27bdffd8 addiu sp,sp,-40
54: ffbf0020 sd ra,32(sp)
58: 04800004 bltz a0,6c <bad_fn3(unsigned)+0x1c>
5c: 0000102d move v0,zero
60: 0c000000 jal 0 <bad_fn1(unsigned)>
60: R_MIPS_26 another_function()
64: 00000000 nop
68: 24020001 li v0,1
6c: dfbf0020 ld ra,32(sp)
70: 03e00008 jr ra
74: 27bd0028 addiu sp,sp,40
0000000000000078 <good_fn1(unsigned)>:
78: 27bdffd8 addiu sp,sp,-40
7c: ffbf0020 sd ra,32(sp)
80: 3c030000 lui v1,0x0
80: R_MIPS_HI16 .data
84: 8c630000 lw v1,0(v1)
84: R_MIPS_LO16 .data
88: 00641823 subu v1,v1,a0
8c: 04610004 bgez v1,a0 <good_fn1(unsigned)+0x28>
90: 0000102d move v0,zero
94: 0c000000 jal 0 <bad_fn1(unsigned)>
94: R_MIPS_26 another_function()
98: 00000000 nop
9c: 24020001 li v0,1
a0: dfbf0020 ld ra,32(sp)
a4: 03e00008 jr ra
a8: 27bd0028 addiu sp,sp,40
ac: 00000000 nop
---------- end of bug.dmp
>How-To-Repeat:
$ ccmips -MD -MP -EL -mcpu=r4000 -mips3 -G 0 -ansi -O1 -Wall -c bug.cpp -o bug.o
$ objdumpmips -drC bug.o > bug.dmp
>Fix:
I can work around the problem by changing the constant 0x80000000 to something else (e.g., 0x80000001). Or, if the constants are put in global variables and cannot be "optimized" at compile time, correct code is generated.
>Release-Note:
>Audit-Trail:
>Unformatted: