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Update doc for C++ TLS calling convention.

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llvm-svn: 254953
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Manman Ren 2015-12-07 21:40:09 +00:00
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docs/LangRef.rst
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@ -407,12 +407,22 @@ added in the future:
used by a future version of the ObjectiveC runtime and should be considered
experimental at this time.
"``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
Clang generates an access function to access C++-style TLS. The access
function generally has an entry block, an exit block and an initialization
block that is run at the first time. The entry and exit blocks can access
a few TLS IR variables, each access will be lowered to a platform-specific
sequence.
This calling convention aims to minimize overhead in the caller by
preserving as many registers as possible. This calling convention behaves
identical to the `C` calling convention on how arguments and return values
are passed, but it uses a different set of caller/callee-saved registers.
Given that C-style TLS on Darwin has its own special CSRs, we can't use the
existing `PreserveMost`.
preserving as many registers as possible (all the registers that are
perserved on the fast path, composed of the entry and exit blocks).
This calling convention behaves identical to the `C` calling convention on
how arguments and return values are passed, but it uses a different set of
caller/callee-saved registers.
Given that each platform has its own lowering sequence, hence its own set
of preserved registers, we can't use the existing `PreserveMost`.
- On X86-64 the callee preserves all general purpose registers, except for
RDI and RAX.