On 2024-08-20 09:09, Xi Ruoyao wrote:
On Tue, 2024-08-20 at 08:50 +0800, Jinyang He wrote:
On 2024-08-19 23:36, Xi Ruoyao wrote:
On Mon, 2024-08-19 at 13:03 +0000, Jason A. Donenfeld wrote:
The compiler (GCC 14.2) calls memset() for initializing a "large" struct
in a cold path of the generic vDSO getrandom() code. There seems no way
to prevent it from calling memset(), and it's a cold path so the
performance does not matter, so just provide a naive memset()
implementation for vDSO.
Why x86 doesn't need to provide a naive memset()?
I'm not sure. Maybe it's because x86_64 has SSE2 enabled so by default
the maximum buffer length to inline memset is larger.
I suspect the loongarch gcc has issue with -fno-builtin(-memset).
No, -fno-builtin-memset just means don't convert memset to
__builtin_memset, it does not mean "don't emit memset call," nor
anything more than that.
Even -ffreestanding is not guaranteed to turn off memset call generation
because per the standard memset should be available even in a
freestanding implementation.
x86 has a -mmemset-strategy= option but it's really x86 specific. As
Jason pointed out, PowerPC and ARM64 have also hit the same issue.
I see. Thanks! The gcc produced __builtin_memset in expand pass.
X86 increase the maximum buffer length to inline memset by
`-mmemset-strategy=`, while other archs like LoongArch cannot do this.
