Subtract with borrow

[Mike Spivey]

The i386 implementation in GIT contains the definition for
subtract-with-carry:

#define jit_subxr_i(d, s1, s2) \
    jit_opr_((d), (s1), (s2), SBBLrr((s1), (d)), SBBLrr((s2), (d)) )

Please forgive my not compiling a test, but doesn't this treat
subtraction as if it is commutative?  Correct code for the case where
s1 != d but s2 = d will be embarrassingly bad, but (if I'm right about
the error) bad code is better than wrong code, always!

Best wishes,

-- Mike



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[Paolo Bonzini-2]

On 08/04/2009 07:19 PM, Mike Spivey wrote:

> The i386 implementation in GIT contains the definition for
> subtract-with-carry:
>
> #define jit_subxr_i(d, s1, s2) \
>      jit_opr_((d), (s1), (s2), SBBLrr((s1), (d)), SBBLrr((s2), (d)) )
>
> Please forgive my not compiling a test, but doesn't this treat
> subtraction as if it is commutative?  Correct code for the case where
> s1 != d but s2 = d will be embarrassingly bad, but (if I'm right about
> the error) bad code is better than wrong code, always!

I think you're right... :-(

(The code wouldn't be so bad, it just needs some push/pops).

Paolo


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[Mike Spivey]

FWIW, here's a test case to verify the bug.  It prints -3 (wring) and
not 3 (correct).

I'm not using Lightning except as a quarry for useful bits of code, so I
won't suggest a fix.

"Bad" is a relative term here, and a register allocator can avoid most
of the bad cases in most instructions by making rd = rs1 wherever
possible.

-- Mike

#include <stdio.h>
#include "lightning.h"

static jit_insn codeBuffer[1024];

typedef int (*pifi)(int, int);

int main()
{
     pifi  subr = (pifi) (jit_set_ip(codeBuffer).iptr);
     int   in1, in2;

     jit_leaf(2);                  
     in1 = jit_arg_i();              
     in2 = jit_arg_i();              
     jit_getarg_i(JIT_R0, in1);
     jit_getarg_i(JIT_R1, in2);
     jit_subcr_i(JIT_R2, JIT_R2, JIT_R2); /* Clear carry */
     jit_subxr_i(JIT_R1, JIT_R0, JIT_R1);
     jit_movr_i(JIT_RET, JIT_R1);
     jit_ret();                    

     jit_flush_code(codeBuffer, jit_get_ip().ptr);

     /* call the generated code, passing 5 as an argument */
     printf("result = %d\n", subr(5, 2));
     return 0;
}


On Wed, 2009-08-05 at 01:10 +0200, Paolo Bonzini wrote:

> On 08/04/2009 07:19 PM, Mike Spivey wrote:
> > The i386 implementation in GIT contains the definition for
> > subtract-with-carry:
> >
> > #define jit_subxr_i(d, s1, s2) \
> >      jit_opr_((d), (s1), (s2), SBBLrr((s1), (d)), SBBLrr((s2), (d)) )
> >
> > Please forgive my not compiling a test, but doesn't this treat
> > subtraction as if it is commutative?  Correct code for the case where
> > s1 != d but s2 = d will be embarrassingly bad, but (if I'm right about
> > the error) bad code is better than wrong code, always!
>
> I think you're right... :-(
>
> (The code wouldn't be so bad, it just needs some push/pops).
>
> Paolo

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