[PATCH] Add a "probe-stack" attribute

[Reid Kleckner]

This seems like a reasonable implementation of useful functionality.

================
Comment at: lib/Target/X86/X86FrameLowering.cpp:675-692
@@ -668,4 +674,20 @@
 
-    if (Is64Bit) {
-      if (STI.isTargetCygMing()) {
-        StackProbeSymbol = "___chkstk_ms";
+    if (NumBytes <= 0x5000) {
+      BuildMI(MBB, MBBI, DL, TII.get(Is64Bit ? X86::SUB64ri32 : X86::SUB32ri),
+              StackPtr)
+          .addReg(StackPtr)
+          .addImm(NumBytes)
+        .setMIFlag(MachineInstr::FrameSetup);
+
+      for (uint64_t i = 0; i < NumBytes / 0x1000; ++i) {
+        BuildMI(MBB, MBBI, DL, TII.get(Is64Bit ? X86::OR64mi8 : X86::OR32mi8))
+          .addReg(StackPtr)
+          .addImm(1)
+          .addReg(0)
+          .addImm(NumBytes - (i + 1) * 0x1000)
+          .addReg(0)
+          .addImm(0)
+          .setMIFlag(MachineInstr::FrameSetup);
+      }
+    } else {
+      const char *StackProbeSymbol;
----------------
This change seems like a separable optimization, correct?  I'd like to have a separate patch for it.

================
Comment at: lib/Target/X86/X86FrameLowering.cpp:752
@@ +751,3 @@
+      if (isRegAccAlive) {
+          // Restore RegAcc
+          auto MIB = BuildMI(MF, DL,
----------------
Should be 2 space indentation

http://reviews.llvm.org/D4717