Support 128-bit atomics on x86_64-fortanix-unknown-sgx

[taiki-e]

Based on my comment in #99069 (comment):

If my understanding about the SGX target is correct, this is a target that requires Intel chip-specific technology, so I suspect that 128-bit atomics is always available (because all Intel x86_64 chips supports cmpxchg16b).

cc @jethrogb: Is it okay to have the above assumptions for this target? (Or is this target intentionally not currently enabling cx16 for some reason?)

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[jethrogb]

Yes SGX is generally considered Intel-specific. If you could point to where in the Intel documentation it talks about 128-bit atomics always being supported, that would be helpful.

[taiki-e]

If you could point to where in the Intel documentation it talks about 128-bit atomics always being supported, that would be helpful.

Sorry, I do not know of any Intel documentation that explains this.

I mainly referred to the following three resources:

• Wikipedia
  https://en.wikipedia.org/wiki/X86-64#Older_implementations

  The AMD64 processors prior to Revision F[57] (distinguished by the switch from DDR to DDR2 memory and new sockets AM2, F and S1) of 2006 lacked the CMPXCHG16B instruction, which is an extension of the CMPXCHG8B instruction present on most post-80486 processors.

• LLVM

  FeatureCX16 says:

  64-bit with cmpxchg16b (this is true for most x86-64 chips, but not the first AMD chips)

  Also, Nocona (FeaturesNocona) enables FeatureCMPXCHG16B, Core2 (FeaturesCore2) enables FeaturesNocona, Bonnell (FeaturesBonnell) enables FeaturesCore2.

  According to Wikipedia, Nocona is the first Intel x64 processor and Core2 is the first Intel x64 mobile processor, Bonnell is the microarchitecture of the first generation of Atom processors.

• GCC

  Both Nocona (PROCESSOR_NOCONA) and Core2 (PROCESSOR_CORE2 via PTA_CORE2) enables PTA_CX16, Bonnell (PROCESSOR_BONNELL via PTA_BONNELL) enables PTA_CORE2.

[taiki-e]

If you could point to where in the Intel documentation it talks about 128-bit atomics always being supported, that would be helpful.

Sorry, I do not know of any Intel documentation that explains this.

Ah, "CMPXCHG8B/CMPXCHG16B—Compare and Exchange Bytes" section in Intel 64 and IA-32 Architectures Software Developer’s Manual says CMPXCHG16B in 64-Bit Mode is "Valid".

(When it is not available on older CPUs, such as CMPXCHG8B/CMPXCHG in Compat/Leg Mode, it has annotation about compatibility -- they are "Valid*" not "Valid" and have "IA-32 Architecture Compatibility" sections that say "not supported on Intel processors earlier than ...".)

EDIT: I missed exceptions sections #130552 (comment)

[jethrogb]

Just the “valid” note is not sufficient. In the instruction description under 64-mode exceptions, it says:

#UD If CPUID.01H:ECX.CMPXCHG16B[bit 13] = 0.

Which indicates that CPU support is not guaranteed in general.