Reuse %rax after calling __chkstk on win64

Reapply this as I reverted the wrong commit.

llvm-svn: 185807

llvm/lib/Target/X86/X86FrameLowering.cpp

@@ -914,11 +914,14 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF) const {
       .addReg(X86::EFLAGS, RegState::Define | RegState::Implicit)
       .setMIFlag(MachineInstr::FrameSetup);
 
-    // MSVC x64's __chkstk needs to adjust %rsp.
-    // FIXME: %rax preserves the offset and should be available.
-    if (isSPUpdateNeeded)
-      emitSPUpdate(MBB, MBBI, StackPtr, -(int64_t)NumBytes, Is64Bit, IsLP64,
-                   UseLEA, TII, *RegInfo);
+    // MSVC x64's __chkstk does not adjust %rsp itself.
+    // It also does not clobber %rax so we can reuse it when adjusting %rsp.
+    if (isSPUpdateNeeded) {
+      BuildMI(MBB, MBBI, DL, TII.get(X86::SUB64rr), StackPtr)
+        .addReg(StackPtr)
+        .addReg(X86::RAX)
+        .setMIFlag(MachineInstr::FrameSetup);
+    }
 
     if (isEAXAlive) {
         // Restore EAX

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -14432,12 +14432,11 @@ X86TargetLowering::EmitLoweredWinAlloca(MachineInstr *MI,
     } else {
       // __chkstk(MSVCRT): does not update stack pointer.
       // Clobbers R10, R11 and EFLAGS.
-      // FIXME: RAX(allocated size) might be reused and not killed.
       BuildMI(*BB, MI, DL, TII->get(X86::W64ALLOCA))
         .addExternalSymbol("__chkstk")
         .addReg(X86::RAX, RegState::Implicit)
         .addReg(X86::EFLAGS, RegState::Define | RegState::Implicit);
-      // RAX has the offset to subtracted from RSP.
+      // RAX has the offset to be subtracted from RSP.
       BuildMI(*BB, MI, DL, TII->get(X86::SUB64rr), X86::RSP)
         .addReg(X86::RSP)
         .addReg(X86::RAX);

llvm/test/CodeGen/X86/win64_alloca_dynalloca.ll

@@ -19,7 +19,7 @@ entry:
 ; W64: movq  %rsp, %rbp
 ; W64:       $4096, %rax
 ; W64: callq __chkstk
-; W64: subq  $4096, %rsp
+; W64: subq  %rax, %rsp
 
 ; Freestanding
 ; EFI: movq  %rsp, %rbp