[llvm-dev] Aggressive optimization opportunity

[Roman Lebedev via llvm-dev]

On Tue, Jan 15, 2019 at 3:08 PM Zheng CZ Chen via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> There are some compilers with a aggressive optimization which restricts function pointer parameters. Let's say opt restrict_args. When restrict_args is turned on, compiler will treat all function pointer parameters as restrict one.
>
> int foo(int * a) + restrict_args opt
>
> equals to:
>
> int foo(int * restrict a)
>
>
> Here is a complete example:
> source code:
> extern int num;
> int foo(int * a)
> {
> (*a) = 10;
> num++;
> (*a)++;
>
> return *a;
> }
>
> Using IBM xlc compiler with option -qrestrict at -O2, we get result:
>
> 0000000000000000 <foo>:
> 0: 00 00 4c 3c addis r2,r12,0
> 4: 00 00 42 38 addi r2,r2,0
> 8: 00 00 a2 3c addis r5,r2,0
> c: 00 00 a5 e8 ld r5,0(r5)
> 10: 0b 00 00 38 li r0,11
> 14: 00 00 03 90 stw r0,0(r3)
> 18: 00 00 85 80 lwz r4,0(r5)
> 1c: 0b 00 60 38 li r3,11 ------>since we confirm num will not change the content where pointer to, compiler can directly return 11.
> 20: 01 00 04 38 addi r0,r4,1
> 24: 00 00 05 90 stw r0,0(r5)
> 28: 20 00 80 4e blr
>
> Seems clang does not have such optimization. And I don't find similar option in gcc either.
>
> Is it possible to add this optimization into clang?
E.g. https://godbolt.org/z/gB98K0

> Thanks.
>
> BRS//
> Chen Zheng
> Power Compiler Backend Developer
Roman.

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