> On 15 Jan 2025, at 08:38, Richard Biener <richard.guent...@gmail.com> wrote:
>
> External email: Use caution opening links or attachments
>
>
> On Tue, Jan 14, 2025 at 4:35 PM Jennifer Schmitz via Gcc
> <gcc@gcc.gnu.org> wrote:
>>
>> We are working on improving codegen for the following test cases (for all
>> integer types T):
>>
>> T foo (T x, T y)
>> {
>> T diff = x - y;
>> return x > y ? diff : -diff;
>> }
>>
>> T bar (T x, T y)
>> {
>> T diff1 = x - y;
>> T diff2 = y - x;
>> return x > y ? diff1 : diff2;
>> }
>>
>> For signed integers, we already proposed a patch (attached to [1]) that
>> amends existing match.pd
>> patterns in order to produce an ABS_EXPR (x - y).
>>
>> Now, we want to implement the optimization for unsigned integers for
>> AArch64. For example,
>> GCC compiles the function bar for uint8_t to (-O3 -fwrapv)
>> bar_u8:
>> and w3, w0, 255
>> and w1, w1, 255
>> sub w2, w3, w1
>> sub w0, w1, w3
>> and w2, w2, 255
>> cmp w3, w1
>> and w0, w0, 255
>> csel w0, w0, w2, ls
>> ret
>>
>> whereas clang produces the desired sequence
>> bar_u8:
>> and w8, w0, #0xff
>> sub w8, w8, w1, uxtb
>> cmp w8, #0
>> cneg w0, w8, mi
>> ret
>>
>> We would like to ask for guidance on where to best implement this
>> optimization for unsigned
>> integers. We have considered the following approaches:
>> - also in match.pd as for the signed integers. However, the existing rule
>> ABS (A) -> A for
>> unsigned integers would fold ABS_EXPR (x - y) to (x - y), which is
>> incorrect if x < y.
>> Are there other ways to express absolute differences for unsigned types on
>> gimple level?
>
> Well, iff x > y ? x - y : y - x -> abs ((signed)(x - y)) == abs
> ((signed)(y - x)) then I'd suggest
> to instead of ABS_EXPR (x - y) use ABSU_EXPR ((signed)(x - y)).
>
> But I'm not sure that's OK when the difference cannot be represented
> in a signed integer, like
> for x == UINT_MAX and y == 0. The desired assembly uses conditional
> negation (foo), so
> why not use that form as target? IIRC value-numbering will already
> try to CSE y - x as -T
> when there's a T == x - y.
Hi Richard,
thanks for responding to my email.
I tried using ABSU_EXPR ((signed)(x - y)), but as you expected it yields
incorrect results
for some corner cases (if the top bit is set in x - y). So, I’m not sure if
there is a
good way to fold it on gimple level that also gives correct results in all
cases.
That points to doing it on RTL level...
Thanks,
Jennifer
>
> Richard.
>
>> - in the aarch64 backend on RTL level: create an if_then_else RTX with
>> (zero-extended) minus expressions
>> in both arms. However, the combine-pass dump shows that we are also lacking
>> an instruction pattern
>> matching each arm:
>> Failed to match this instruction:
>> (set (reg:SI 108)
>> (zero_extend:SI (minus:QI (subreg:QI (reg/v:SI 103 [ x ]) 0)
>> (subreg:QI (reg/v:SI 104 [ y ]) 0))))
>> which we would want to map to the following split of 3 instructions:
>> and r103, r103, 255
>> sub r108, r103, r104, uxtb
>> and r108, r108, 255
>> Do we want to add both those patterns?
>>
>> Any advice would be appreciated.
>> Thanks,
>> Jennifer
>>
>> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114999
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