X86: Use a smaller 'mov' instruction for stack probe calls

Prologue emission, in some cases, requires calls to a stack probe helper
function.  The amount of stack to probe is passed as a register
argument in the Win64 ABI but the instruction sequence used is
pessimistic: it assumes that the number of bytes to probe is greater
than 4 GB.

Instead, select a more appropriate opcode depending on the number of
bytes we are going to probe.

llvm-svn: 230270

1 files changed, 13 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index 75fd4fdb434..cf7a4ce4bed 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -820,9 +820,19 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF) const {
     if (Is64Bit) {
       // Handle the 64-bit Windows ABI case where we need to call __chkstk.
       // Function prologue is responsible for adjusting the stack pointer.
-      BuildMI(MBB, MBBI, DL, TII.get(X86::MOV64ri), X86::RAX)
-        .addImm(NumBytes)
-        .setMIFlag(MachineInstr::FrameSetup);
+      if (isUInt<32>(NumBytes)) {
+        BuildMI(MBB, MBBI, DL, TII.get(X86::MOV32ri), X86::EAX)
+            .addImm(NumBytes)
+            .setMIFlag(MachineInstr::FrameSetup);
+      } else if (isInt<32>(NumBytes)) {
+        BuildMI(MBB, MBBI, DL, TII.get(X86::MOV64ri32), X86::RAX)
+            .addImm(NumBytes)
+            .setMIFlag(MachineInstr::FrameSetup);
+      } else {
+        BuildMI(MBB, MBBI, DL, TII.get(X86::MOV64ri), X86::RAX)
+            .addImm(NumBytes)
+            .setMIFlag(MachineInstr::FrameSetup);
+      }
     } else {
       // Allocate NumBytes-4 bytes on stack in case of isEAXAlive.
       // We'll also use 4 already allocated bytes for EAX.