Combiner fixes

[Bernd Schmidt]

On 08/03/2010 09:24 AM, Paolo Bonzini wrote:
> On 08/02/2010 10:37 PM, Bernd Schmidt wrote:
>> +      if (GET_CODE (op0) == NEG && CONST_INT_P (trueop1))
>> +    return simplify_gen_binary (MULT, mode, XEXP (op0, 0),
>> +                    simplify_gen_unary (NEG, mode, op1, mode));
> 
> Why not go one step further and try it on all operands:
> 
>   if (GET_CODE (op0) == NEG)
>     {
>       rtx temp = simplify_unary (NEG, mode, op1, mode);
>       if (temp)
>         return simplify_gen_binary (MULT, mode, XEXP (op0, 0), temp);
>     }
>   if (GET_CODE (op1) == NEG)
>     {
>       rtx temp = simplify_unary (NEG, mode, op0, mode);
>       if (temp)
>         return simplify_gen_binary (MULT, mode, temp, XEXP (op1, 0));
>     }

Done (slight typo in the above, needs simplify_unary_operation), and
also implemented the opposite transformation in combine:
 (minus x (mult y -12345))
becomes
 (plus (mult y 12345) x)

I've now also looked at code generation on i686, where it also seems to
help occasionally:
-       imull   $-12, 4(%ecx), %edx
-       movl    $4, %eax
-       subl    %edx, %eax
+       imull   $12, 4(%ecx), %eax
+       addl    $4, %eax
=========
-       sall    $5, %eax
-       negl    %eax
-       imull   $-2, %eax, %eax
+       sall    $6, %eax

There's a single counterexample I found, in 20040709-2.c:
-       imull   $-1029531031, %ecx, %ebp
-       subl    $740551042, %ebp
+       imull   $1103515245, %ecx, %ebp
+       addl    $12345, %ebp
+       imull   $1103515245, %ebp, %ebp
+       addl    $12345, %ebp

where an intermediate (minus (const) (mult x const)) is not recognized
as a valid pattern in combine, which then prevents later
transformations.  I think it's one of these cases where combine could
benefit from looking at 4 insns.

Bootstrapped and regression tested on i686-linux.  In the ARM tests,
with the previous patch I saw an intermittent segfault on one testcase,
which wasn't reproducible when running the compiler manually, and has
gone away with the new version (tests still running).  I think it's
unrelated.


Bernd
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