[llvm-dev] [RFC] Spill2Reg: Selectively replace spills to stack with spills to vector registers

[Craig Topper via llvm-dev]

On Thu, Jan 27, 2022 at 11:25 AM Vasileios Porpodas via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Momchil,
> Spill2Reg definitely adds to the FP context, but I am not sure how it
> would interact with lazy save/restore.
> Vasileios
>

The XSAVE instructions allow the OS to skip restoring the XMM registers on
context switches if they are in their initial state or haven't changed.
Spilling to XMM registers will make the registers dirty in more cases than
they might have been otherwise.

That reminds me that this needs to be disabled when the NoImplicitFloat
attribute is present. One use for NoImplicitFloat is to prevent uses of FP
and vector registers when compiling OS kernels. If the kernel code doesn't
change the FP or vector registers, they don't need to be
saved/restored during system calls. I hope you already took into account
-mno-sse/-no-sse2.

~Craig


>
> On Thu, Jan 27, 2022 at 9:34 AM Momchil Velikov <momchil.velikov at gmail.com>
> wrote:
>
>> It's interesting how this interacts with lazy save/restore of the FP
>> context across context switches.
>>
>> ~chill
>> --
>> Compiler scrub, Arm
>>
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