[X86] Merge adjacent stack adjustments in eliminateCallFramePseudoInstr (PR27140)

For code such as:

  void f(int, int);
  void g() {
      f(1, 2);
  }

compiled for 32-bit X86 Linux, Clang would previously generate:

  subl    $12, %esp
  subl    $8, %esp
  pushl   $2
  pushl   $1
  calll   f
  addl    $16, %esp
  addl    $12, %esp
  retl

This patch fixes that by merging adjacent stack adjustments in
eliminateCallFramePseudoInstr().

Differential Revision: http://reviews.llvm.org/D18627

llvm-svn: 265039

1 files changed, 19 insertions, 12 deletions

diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index 413195f09c6..d8a376a6200 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -2534,13 +2534,22 @@ eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
       BuildCFI(MBB, I, DL, 
                MCCFIInstruction::createAdjustCfaOffset(nullptr, -InternalAmt));
 
-    if (Amount) {
-      // Add Amount to SP to destroy a frame, and subtract to setup.
-      int Offset = isDestroy ? Amount : -Amount;
+    // Add Amount to SP to destroy a frame, or subtract to setup.
+    int64_t StackAdjustment = isDestroy ? Amount : -Amount;
 
-      if (!(Fn->optForMinSize() && 
-            adjustStackWithPops(MBB, I, DL, Offset)))
-        BuildStackAdjustment(MBB, I, DL, Offset, /*InEpilogue=*/false);
+    if (StackAdjustment) {
+      // Merge with any previous or following adjustment instruction.
+      StackAdjustment += mergeSPUpdates(MBB, I, true);
+      StackAdjustment += mergeSPUpdates(MBB, I, false);
+
+      if (!StackAdjustment) {
+        // This and the merged instruction canceled out each other.
+        return I;
+      }
+
+      if (!(Fn->optForMinSize() &&
+            adjustStackWithPops(MBB, I, DL, StackAdjustment)))
+        BuildStackAdjustment(MBB, I, DL, StackAdjustment, /*InEpilogue=*/false);
     }
 
     if (DwarfCFI && !hasFP(MF)) {
@@ -2550,14 +2559,12 @@ eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
       // CFI only for EH purposes or for debugging. EH only requires the CFA
       // offset to be correct at each call site, while for debugging we want
       // it to be more precise.
-      int CFAOffset = Amount;
+
       // TODO: When not using precise CFA, we also need to adjust for the
       // InternalAmt here.
-
-      if (CFAOffset) {
-        CFAOffset = isDestroy ? -CFAOffset : CFAOffset;
-        BuildCFI(MBB, I, DL, 
-                 MCCFIInstruction::createAdjustCfaOffset(nullptr, CFAOffset));
+      if (StackAdjustment) {
+        BuildCFI(MBB, I, DL, MCCFIInstruction::createAdjustCfaOffset(
+                                 nullptr, -StackAdjustment));
       }
     }