Reduce the size of MCRelaxableFragment.

MCRelaxableFragment previously kept a copy of MCSubtargetInfo and
MCInst to enable re-encoding the MCInst later during relaxation. A copy
of MCSubtargetInfo (instead of a reference or pointer) was needed
because the feature bits could be modified by the parser.

This commit replaces the MCSubtargetInfo copy in MCRelaxableFragment
with a constant reference to MCSubtargetInfo. The copies of
MCSubtargetInfo are kept in MCContext, and the target parsers are now
responsible for asking MCContext to provide a copy whenever the feature
bits of MCSubtargetInfo have to be toggled.
 
With this patch, I saw a 4% reduction in peak memory usage when I
compiled verify-uselistorder.lto.bc using llc.

rdar://problem/21736951

Differential Revision: http://reviews.llvm.org/D14346

llvm-svn: 253127

1 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp b/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
index 234b3b64707..4e4b47805cd 100644
--- a/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
@@ -12,15 +12,21 @@
 using namespace llvm;
 
 MCTargetAsmParser::MCTargetAsmParser(MCTargetOptions const &MCOptions,
-                                     MCSubtargetInfo &STI)
+                                     const MCSubtargetInfo &STI)
   : AvailableFeatures(0), ParsingInlineAsm(false), MCOptions(MCOptions),
-    STI(STI)
+    STI(&STI)
 {
 }
 
 MCTargetAsmParser::~MCTargetAsmParser() {
 }
 
+MCSubtargetInfo &MCTargetAsmParser::copySTI() {
+  MCSubtargetInfo &STICopy = getContext().getSubtargetCopy(getSTI());
+  STI = &STICopy;
+  return STICopy;
+}
+
 const MCSubtargetInfo &MCTargetAsmParser::getSTI() const {
-  return STI;
+  return *STI;
 }