From: Anton Korobeynikov <asl@math.spbu.ru>
Date: Sat, 6 Mar 2010 20:07:32 +0000 (+0000)
Subject: Describe what's going on with mingw alloca and why do we need separate instruction.
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=e765f2b5e3f260c96579859f503a4e5e76cb4c1c;p=oota-llvm.git

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97888 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/X86/X86InstrInfo.td b/lib/Target/X86/X86InstrInfo.td
index 9953b052cf6..8a6ff54c771 100644
--- a/lib/Target/X86/X86InstrInfo.td
+++ b/lib/Target/X86/X86InstrInfo.td
@@ -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)]>;
 }