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GlobalDCE: Improve performance for large modules containing comdats.

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When we encounter a global with a comdat, rather than iterating over
every global in the module to find globals in the same comdat, store the
members in a multimap. This effectively lowers the complexity to O(N log N),
improving performance significantly for large modules such as might be
encountered during LTO.

It looks like we used to do something like this until r219191.

No functional change.

Differential Revision: http://reviews.llvm.org/D8431

llvm-svn: 232743
This commit is contained in:
Peter Collingbourne 2015-03-19 18:23:29 +00:00
parent b8431e70e2
commit c3999ab5f3
1 changed files with 16 additions and 10 deletions
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26
lib/Transforms/IPO/GlobalDCE.cpp
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@ -24,6 +24,7 @@
#include "llvm/Transforms/Utils/CtorUtils.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"
#include "llvm/Pass.h"
#include <unordered_map>
using namespace llvm;
#define DEBUG_TYPE "globaldce"
@ -47,6 +48,7 @@ namespace {
private:
SmallPtrSet<GlobalValue*, 32> AliveGlobals;
SmallPtrSet<Constant *, 8> SeenConstants;
std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
/// GlobalIsNeeded - mark the specific global value as needed, and
/// recursively mark anything that it uses as also needed.
@ -78,6 +80,17 @@ bool GlobalDCE::runOnModule(Module &M) {
// Remove empty functions from the global ctors list.
Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
// Collect the set of members for each comdat.
for (Function &F : M)
if (Comdat *C = F.getComdat())
ComdatMembers.insert(std::make_pair(C, &F));
for (GlobalVariable &GV : M.globals())
if (Comdat *C = GV.getComdat())
ComdatMembers.insert(std::make_pair(C, &GV));
for (GlobalAlias &GA : M.aliases())
if (Comdat *C = GA.getComdat())
ComdatMembers.insert(std::make_pair(C, &GA));
// Loop over the module, adding globals which are obviously necessary.
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
Changed |= RemoveUnusedGlobalValue(*I);
@ -177,6 +190,7 @@ bool GlobalDCE::runOnModule(Module &M) {
// Make sure that all memory is released
AliveGlobals.clear();
SeenConstants.clear();
ComdatMembers.clear();
return Changed;
}
@ -188,17 +202,9 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
if (!AliveGlobals.insert(G).second)
return;
Module *M = G->getParent();
if (Comdat *C = G->getComdat()) {
for (Function &F : *M)
if (F.getComdat() == C)
GlobalIsNeeded(&F);
for (GlobalVariable &GV : M->globals())
if (GV.getComdat() == C)
GlobalIsNeeded(&GV);
for (GlobalAlias &GA : M->aliases())
if (GA.getComdat() == C)
GlobalIsNeeded(&GA);
for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
GlobalIsNeeded(CM.second);
}
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {