[MC] Adding code padding for performance stability - infrastructure. NFC.

Infrastructure designed for padding code with nop instructions in key places such that preformance improvement will be achieved.
The infrastructure is implemented such that the padding is done in the Assembler after the layout is done and all IPs and alignments are known.
This patch by itself in a NFC. Future patches will make use of this infrastructure to implement required policies for code padding.

Reviewers:
aaboud
zvi
craig.topper
gadi.haber

Differential revision: https://reviews.llvm.org/D34393

Change-Id: I92110d0c0a757080a8405636914a93ef6f8ad00e
llvm-svn: 316413

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 2fd969de296..f226c2f0a30 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -108,6 +108,16 @@ MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() {
   return F;
 }
 
+MCPaddingFragment *MCObjectStreamer::getOrCreatePaddingFragment() {
+  MCPaddingFragment *F =
+      dyn_cast_or_null<MCPaddingFragment>(getCurrentFragment());
+  if (!F) {
+    F = new MCPaddingFragment();
+    insert(F);
+  }
+  return F;
+}
+
 void MCObjectStreamer::visitUsedSymbol(const MCSymbol &Sym) {
   Assembler->registerSymbol(Sym);
 }
@@ -247,6 +257,13 @@ bool MCObjectStreamer::mayHaveInstructions(MCSection &Sec) const {
 
 void MCObjectStreamer::EmitInstruction(const MCInst &Inst,
                                        const MCSubtargetInfo &STI, bool) {
+  getAssembler().getBackend().handleCodePaddingInstructionBegin(Inst);
+  EmitInstructionImpl(Inst, STI);
+  getAssembler().getBackend().handleCodePaddingInstructionEnd(Inst);
+}
+
+void MCObjectStreamer::EmitInstructionImpl(const MCInst &Inst,
+                                           const MCSubtargetInfo &STI) {
   MCStreamer::EmitInstruction(Inst, STI);
 
   MCSection *Sec = getCurrentSectionOnly();
@@ -467,6 +484,16 @@ void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset,
   insert(new MCOrgFragment(*Offset, Value, Loc));
 }
 
+void MCObjectStreamer::EmitCodePaddingBasicBlockStart(
+    const MCCodePaddingContext &Context) {
+  getAssembler().getBackend().handleCodePaddingBasicBlockStart(this, Context);
+}
+
+void MCObjectStreamer::EmitCodePaddingBasicBlockEnd(
+    const MCCodePaddingContext &Context) {
+  getAssembler().getBackend().handleCodePaddingBasicBlockEnd(Context);
+}
+
 // Associate DTPRel32 fixup with data and resize data area
 void MCObjectStreamer::EmitDTPRel32Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();