Reduce the number of attributes attached to each function

[xbolva00]

Summary

Patch takes advantage of the implicit default behavior to reduce the number of attributes, which in turns reduces compilation time.

[serge-sans-paille]

I checked the usage of this attribute, and it's indeed ignored when set to zero, so yeah, let's just not generate it in that case.

[crownyanguan]

However, this commit will cause performance regression.

[xbolva00]

In D97116#2596275, @crownyanguan wrote:

However, this commit will cause performance regression.

More details?

[crownyanguan]

In D97116#2596409, @xbolva00 wrote:

In D97116#2596275, @crownyanguan wrote:

However, this commit will cause performance regression.

More details?

This commit will decrease the frequence of intel CPU, causing a performance regression.

[lebedev.ri]

In D97116#2610652, @crownyanguan wrote:

In D97116#2596409, @xbolva00 wrote:

In D97116#2596275, @crownyanguan wrote:

However, this commit will cause performance regression.

More details?

This commit will decrease the frequence of intel CPU, causing a performance regression.

You'll have to be a little bit less succinct than that i'm afraid.

[wxiao3]

clang/lib/CodeGen/CodeGenFunction.cpp

496

For x86 backend, "LargestVectorWidth" default value is not zero but UINT32_MAX.
Please refer to: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/X86/X86TargetMachine.cpp#L271
Suggest revert this patch since it changes the code generated for x86.

[lebedev.ri]

clang/lib/CodeGen/CodeGenFunction.cpp

496

Reproducer please?

[wxiao3]

clang/lib/CodeGen/CodeGenFunction.cpp

496

$ cat test.c
void foo(float* in, float* __restrict out, int n)
{
    int i;
    for (i = 0; i < n; i++) {
        out[i] = i + 2*in[i];
    }
}

Before this patch

$ clang -Ofast -march=skylake-avx512 -mllvm -force-vector-width=16 -mprefer-vector-width=256 -S -o - test.c | grep ymm | wc
     57     333    2687

After this patch

$ clang -Ofast -march=skylake-avx512 -mllvm -force-vector-width=16 -mprefer-vector-width=256 -S -o - test.c | grep ymm | wc
      0       0       0

[xbolva00]

clang/lib/CodeGen/CodeGenFunction.cpp

496

Hello, reverted. https://reviews.llvm.org/rG2cb8c10342ee5d040725abb1166feb92a64c7df6

It would be good to somehow make a test to not regress in the future, but not sure how, as we have no "C -> X86 asm" tests?