ARM: add VLA extension for WoA Itanium ABI

The armv7-windows-itanium environment is nearly identical to the MSVC ABI. It
has a few divergences, mostly revolving around the use of the Itanium ABI for
C++. VLA support is one of the extensions that are amongst the set of the
extensions.

This adds support for proper VLA emission for this environment. This is
somewhat similar to the handling for __chkstk emission on X86 and the large
stack frame emission for ARM. The invocation style for chkstk is still
controlled via the -mcmodel flag to clang.

Make an explicit note that this is an extension.

llvm-svn: 210489

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMInstrInfo.td b/llvm/lib/Target/ARM/ARMInstrInfo.td
index 718d5da9d05..b44a19b01c5 100644
--- a/llvm/lib/Target/ARM/ARMInstrInfo.td
+++ b/llvm/lib/Target/ARM/ARMInstrInfo.td
@@ -5093,6 +5093,19 @@ def MSRi : ABI<0b0011, (outs), (ins msr_mask:$mask,  so_imm:$a), NoItinerary,
   let Inst{11-0} = a;
 }
 
+// Dynamic stack allocation yields a _chkstk for Windows targets.  These calls
+// are 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 win__chkstk : SDNode<"ARMISD::WIN__CHKSTK", SDTNone,
+                      [SDNPHasChain, SDNPSideEffect]>;
+let usesCustomInserter = 1, Uses = [R4], Defs = [R4, SP] in
+  def WIN__CHKSTK : PseudoInst<(outs), (ins), NoItinerary, [(win__chkstk)]>;
+
 //===----------------------------------------------------------------------===//
 // TLS Instructions
 //