On Fri, Jul 1, 2011 at 1:58 PM, Stubbs, Andrew <andrew_stu...@mentor.com> wrote:
> On 28/06/11 17:37, Michael Matz wrote:
>>> What I want (and I'm not totally clear on what this actually means) is
>>> > to be able to optimize all the cases where the end result will be the
>>> > same as the compiler produces now (using multiple multiply, shift, and
>>> > add operations).
>> Okay, then you really want to look through value-preserving conversions.
>>
>>> > Ok, so that's an obvious statement, but the point is that, right now,
>>> > the compiler does nothing special when you cast from int -> unsigned
>>> > int, or vice-versa, and I want to capture that somehow. There are some
>>> > exceptions, I'm sure, but what are they?
>> Same-sized signed<-> unsigned conversions aren't value preserving:
>> unsigned char c = 255; (signed char)c == -1; 255 != -1
>> unsigned -> larger sized signed is value preserving
>> unsigned char c = 255; (signed short)c == 255;
>> signed -> unsigned never is value preserving
>
> OK, so I've tried implementing this, and I find I hit against a problem:
>
> Given this test case:
>
> unsigned long long
> foo (unsigned long long a, signed char *b, signed char *c)
> {
> return a + *b * *c;
> }
>
> Those rules say that it should not be suitable for optimization because
> there's an implicit cast from signed int to unsigned long long.
>
> Without any widening multiplications allowed, GCC gives this code (for ARM):
>
> ldrsb r2, [r2, #0]
> ldrsb r3, [r3, #0]
> mul r2, r2, r3
> adds r0, r0, r2
> adc r1, r1, r2, asr #31
>
> This is exactly what a signed widening multiply-and-accumulate with
> smlalbb would have done!
>
> OK, so the types in the testcase are a bit contrived, but my point is
> that I want to be able to use the widening-mult instructions everywhere
> that they would produce the same output and gcc would otherwise, and gcc
> just doesn't seem that interested in signed<->unsigned conversions.
>
> So, I'm happy to put in checks to ensure that truncations are not
> ignore, but I'm really not sure what's the right thing to do with the
> extends and signedness switches.
>
> Any suggestions?
Well - some operations work the same on both signedness if you
just care about the twos-complement result. This includes
multiplication (but not for example division). For this special
case I suggest to not bother trying to invent a generic predicate
but do something local in tree-ssa-math-opts.c.
Richard.
> Andrew
>
