Some implementations can use the std::nullopt_t constructor of
std::optional to avoid needing to completely zero out the internal
buffer of the optional and instead only set the validity byte within it.
e.g. Consider the following function:
std::optional<std::vector<ShaderDiskCacheRaw>> fn() {
return {};
}
With libc++ this will result in the following code generation on x86-64:
Fn():
mov rax, rdi
vxorps xmm0, xmm0, xmm0
vmovups ymmword ptr [rdi], ymm0
vzeroupper
ret
With libstdc++, we also get the similar equivalent:
Fn():
vpxor xmm0, xmm0, xmm0
mov rax, rdi
vmovdqu XMMWORD PTR [rdi], xmm0
vmovdqu XMMWORD PTR [rdi+16], xmm0
ret
If we change this function to return std::nullopt instead, then this
simplifies both the code gen from libc++ and libstdc++ down to:
Fn():
mov BYTE PTR [rdi+24], 0
mov rax, rdi
ret
Given how little of a change is necessary to result in better code
generation, this is essentially a "free" very minor optimization.
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