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, 5 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index 2160b4ee80d..93c7aaf36ee 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -78,6 +78,7 @@ void MCContext::reset() {
   ELFAllocator.DestroyAll();
   MachOAllocator.DestroyAll();
 
+  MCSubtargetAllocator.DestroyAll();
   UsedNames.clear();
   Symbols.clear();
   SectionSymbols.clear();
@@ -439,6 +440,10 @@ MCSectionCOFF *MCContext::getAssociativeCOFFSection(MCSectionCOFF *Sec,
                         COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE);
 }
 
+MCSubtargetInfo &MCContext::getSubtargetCopy(const MCSubtargetInfo &STI) {
+  return *new (MCSubtargetAllocator.Allocate()) MCSubtargetInfo(STI);
+}
+
 //===----------------------------------------------------------------------===//
 // Dwarf Management
 //===----------------------------------------------------------------------===//