* [9fans] 8c vlong bug
@ 2009-09-01 1:47 erik quanstrom
2009-09-01 1:51 ` Bruce Ellis
0 siblings, 1 reply; 4+ messages in thread
From: erik quanstrom @ 2009-09-01 1:47 UTC (permalink / raw)
To: 9fans
temporarly out of time on this one. it appears
from the assembly output that 8c multiplies by
0 and not 1 when computing z a second time.
nonetheless, i haven't yet seen the problem.
#include <u.h>
#include <libc.h>
void
main(void)
{
int three, one;
uvlong twelve, z;
one = 1;
three = 3;
twelve = 12;
z = one*(twelve - three);
print("z = %llud\n", z);
z = (twelve - three) * one;
print("z = %llud\n", z);
exits(0);
}
output:
z = 9
z = 0
- erik
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [9fans] 8c vlong bug 2009-09-01 1:47 [9fans] 8c vlong bug erik quanstrom @ 2009-09-01 1:51 ` Bruce Ellis 2009-09-01 3:10 ` Russ Cox 0 siblings, 1 reply; 4+ messages in thread From: Bruce Ellis @ 2009-09-01 1:51 UTC (permalink / raw) To: Fans of the OS Plan 9 from Bell Labs if it's my fault i'll fix it. it did screw up mod in a subtle way but that's been fixed. brucee On Tue, Sep 1, 2009 at 11:47 AM, erik quanstrom<quanstro@quanstro.net> wrote: > temporarly out of time on this one. it appears > from the assembly output that 8c multiplies by > 0 and not 1 when computing z a second time. > nonetheless, i haven't yet seen the problem. > > #include <u.h> > #include <libc.h> > > void > main(void) > { > int three, one; > uvlong twelve, z; > > one = 1; > three = 3; > twelve = 12; > > z = one*(twelve - three); > print("z = %llud\n", z); > > z = (twelve - three) * one; > print("z = %llud\n", z); > > exits(0); > } > > output: > > z = 9 > z = 0 > > - erik > > ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [9fans] 8c vlong bug 2009-09-01 1:51 ` Bruce Ellis @ 2009-09-01 3:10 ` Russ Cox 2009-09-01 3:24 ` Bruce Ellis 0 siblings, 1 reply; 4+ messages in thread From: Russ Cox @ 2009-09-01 3:10 UTC (permalink / raw) To: Fans of the OS Plan 9 from Bell Labs > temporarly out of time on this one. it appears > from the assembly output that 8c multiplies by > 0 and not 1 when computing z a second time. > nonetheless, i haven't yet seen the problem. not quite that simple. it's a register allocation (really register management) bug. On Mon, Aug 31, 2009 at 6:51 PM, Bruce Ellis<bruce.ellis@gmail.com> wrote: > if it's my fault i'll fix it. it did screw up mod in a subtle way but > that's been fixed. it looks like the cgen64 multiply case is not managing the registers correctly when AX is not where it expects. more details for you: int three = 3; int one = 1; uvlong twelve = 12; /* ok */ uvlong mul1(void) { return one * (twelve - three); } TEXT mul1+0(SB),0,$12 MOVL three+0(SB),AX MOVL AX,CX SARL $31,CX // three is now in CX:AX LEAL twelve+0(SB),DX MOVL (DX),BX SUBL AX,BX MOVL 4(DX),AX SBBL CX,AX // twelve - three is now in AX:BX MOVL AX,CX // twelve - three is now in CX:BX MOVL one+0(SB),AX MOVL AX,DX SARL $31,DX // one is now in DX:AX // 64-bit multiply CX:BX * DX:AX -> DX:AX MOVL CX,BP ORL DX,BP CMPL BP,$0 JNE ,3(PC) MULL BX, JMP ,8(PC) IMULL BX,DX MOVL AX,BP IMULL CX,BP ADDL DX,BP MOVL BX,DX MULL DX, ADDL BP,DX MOVL .ret+0(FP),CX // probably LEAL MOVL AX,(CX) MOVL DX,4(CX) RET , RET , vs /* not ok */ uvlong mul2(void) { return (twelve - three) * one; } TEXT mul2+0(SB),0,$12 MOVL one+0(SB),AX MOVL AX,CX SARL $31,CX MOVL AX,BX // one is now in CX:BX MOVL three+0(SB),AX MOVL AX,DX SARL $31,DX // three is now in DX:AX LEAL twelve+0(SB),BP MOVL (BP),SI SUBL AX,SI MOVL 4(BP),AX SBBL DX,AX // twelve - three is now in AX:SI // 64-bit multiply CX:BX * AX:SI // saving AX and DX so they can be reused. // DX doesn't even need saving. MOVL AX,.safe+-4(SP) MOVL DX,.safe+-8(SP) MOVL SI,DX // 8c is very confused at this point. // it is using AX for multiple purposes. MOVL CX,AX ORL SI,AX CMPL AX,$0 JNE ,4(PC) MOVL BX,AX MULL AX, JMP ,7(PC) IMULL BX,DX IMULL CX,AX ADDL DX,AX MOVL BX,DX MULL DX, ADDL AX,DX // restore old AX DX now that multiply is over. // (forgot that multiply left result in DX:AX) MOVL .safe+-4(SP),AX MOVL .safe+-8(SP),DX MOVL .ret+0(FP),CX // probably LEAL MOVL AX,(CX) MOVL DX,4(CX) RET , ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [9fans] 8c vlong bug 2009-09-01 3:10 ` Russ Cox @ 2009-09-01 3:24 ` Bruce Ellis 0 siblings, 0 replies; 4+ messages in thread From: Bruce Ellis @ 2009-09-01 3:24 UTC (permalink / raw) To: Fans of the OS Plan 9 from Bell Labs will fix. On Tue, Sep 1, 2009 at 1:10 PM, Russ Cox<rsc@swtch.com> wrote: >> temporarly out of time on this one. it appears >> from the assembly output that 8c multiplies by >> 0 and not 1 when computing z a second time. >> nonetheless, i haven't yet seen the problem. > > not quite that simple. > it's a register allocation > (really register management) bug. > > On Mon, Aug 31, 2009 at 6:51 PM, Bruce Ellis<bruce.ellis@gmail.com> wrote: >> if it's my fault i'll fix it. it did screw up mod in a subtle way but >> that's been fixed. > > it looks like the cgen64 multiply case is > not managing the registers correctly > when AX is not where it expects. > more details for you: > > int three = 3; > int one = 1; > uvlong twelve = 12; > > /* ok */ > uvlong > mul1(void) > { > return one * (twelve - three); > } > > TEXT mul1+0(SB),0,$12 > MOVL three+0(SB),AX > MOVL AX,CX > SARL $31,CX > // three is now in CX:AX > > LEAL twelve+0(SB),DX > MOVL (DX),BX > SUBL AX,BX > MOVL 4(DX),AX > SBBL CX,AX > // twelve - three is now in AX:BX > MOVL AX,CX > // twelve - three is now in CX:BX > > MOVL one+0(SB),AX > MOVL AX,DX > SARL $31,DX > // one is now in DX:AX > > // 64-bit multiply CX:BX * DX:AX -> DX:AX > MOVL CX,BP > ORL DX,BP > CMPL BP,$0 > JNE ,3(PC) > MULL BX, > JMP ,8(PC) > IMULL BX,DX > MOVL AX,BP > IMULL CX,BP > ADDL DX,BP > MOVL BX,DX > MULL DX, > ADDL BP,DX > > MOVL .ret+0(FP),CX // probably LEAL > MOVL AX,(CX) > MOVL DX,4(CX) > RET , > RET , > > vs > > /* not ok */ > > uvlong > mul2(void) > { > return (twelve - three) * one; > } > > TEXT mul2+0(SB),0,$12 > MOVL one+0(SB),AX > MOVL AX,CX > SARL $31,CX > MOVL AX,BX > // one is now in CX:BX > > MOVL three+0(SB),AX > MOVL AX,DX > SARL $31,DX > // three is now in DX:AX > > LEAL twelve+0(SB),BP > MOVL (BP),SI > SUBL AX,SI > MOVL 4(BP),AX > SBBL DX,AX > // twelve - three is now in AX:SI > > // 64-bit multiply CX:BX * AX:SI > // saving AX and DX so they can be reused. > // DX doesn't even need saving. > MOVL AX,.safe+-4(SP) > MOVL DX,.safe+-8(SP) > MOVL SI,DX > > // 8c is very confused at this point. > // it is using AX for multiple purposes. > MOVL CX,AX > ORL SI,AX > CMPL AX,$0 > JNE ,4(PC) > MOVL BX,AX > MULL AX, > JMP ,7(PC) > IMULL BX,DX > IMULL CX,AX > ADDL DX,AX > MOVL BX,DX > MULL DX, > ADDL AX,DX > > // restore old AX DX now that multiply is over. > // (forgot that multiply left result in DX:AX) > MOVL .safe+-4(SP),AX > MOVL .safe+-8(SP),DX > > MOVL .ret+0(FP),CX // probably LEAL > MOVL AX,(CX) > MOVL DX,4(CX) > RET , > > ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2009-09-01 3:24 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2009-09-01 1:47 [9fans] 8c vlong bug erik quanstrom 2009-09-01 1:51 ` Bruce Ellis 2009-09-01 3:10 ` Russ Cox 2009-09-01 3:24 ` Bruce Ellis
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