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Describe what's going on with mingw alloca and why do we need separate instruction.

llvm-svn: 97888
This commit is contained in:
Anton Korobeynikov 2010-03-06 20:07:32 +00:00
parent e0e616a74d
commit f6f8265c1c

View File

@ -533,8 +533,16 @@ def VASTART_SAVE_XMM_REGS : I<0, Pseudo,
imm:$regsavefi,
imm:$offset)]>;
// Dynamic stack allocation yields _alloca call for Cygwin/Mingw targets. Calls
// to _alloca is needed to probe the stack when allocating more than 4k bytes in
// one go. Touching the stack at 4K increments is necessary to ensure that the
// guard pages used by the OS virtual memory manager are allocated in correct
// sequence.
// The main point of having separate instruction are extra unmodelled effects
// (compared to ordinary calls) like stack pointer change.
def MINGW_ALLOCA : I<0, Pseudo, (outs), (ins),
"call __alloca",
"# dynamic stack allocation",
[(X86MingwAlloca)]>;
}