[NFC] ConstantMerge: don't insert when find should be used

[jfb]

Summary

DenseMap's operator[] performs an insertion if the entry isn't found. The second phase of ConstantMerge isn't trying to insert anything: it's just looking to see if the first phased performed an insertion. Use find instead, avoiding insertion of every single global initializer in the map of constants. This has the side-effect of making all entries in CMap non-null (because only global declarations would have null initializers, and that would be a bug).

[jfb]

To clarify: my intent is that this change be NFC. Updating the title accordingly.

[mehdi_amini]

LGTM

[davide]

LGTM as discussed in person.

[dblaikie]

You could use D.count(V) to save the comparison with end, etc.

[jfb]

In D50476#1197364, @dblaikie wrote:

You could use D.count(V) to save the comparison with end, etc.

I don't understand what you're suggesting, could you clarify?

[dblaikie]

In D50476#1197511, @jfb wrote:

In D50476#1197364, @dblaikie wrote:

You could use D.count(V) to save the comparison with end, etc.

I don't understand what you're suggesting, could you clarify?

Oh, read the code more - my mistake. Thought you were only testing for whether something was in the container - not actually having to use it afterwards. Carry on!

[jfb]

In D50476#1197515, @dblaikie wrote:

In D50476#1197511, @jfb wrote:

In D50476#1197364, @dblaikie wrote:

You could use D.count(V) to save the comparison with end, etc.

I don't understand what you're suggesting, could you clarify?

Oh, read the code more - my mistake. Thought you were only testing for whether something was in the container - not actually having to use it afterwards. Carry on!

Ah OK! No worries, I thought *I* was having Monday-grogginess issues :-)

lib/Transforms/IPO/ConstantMerge.cpp

for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
 
       // We can only replace constant with local linkage.
       if (!GV->hasLocalLinkage())
         continue;
 
       Constant *Init = GV->getInitializer();
 
       // Check to see if the initializer is already known.
-      GlobalVariable *Slot = CMap[Init];
+      auto Found = CMap.find(Init);
+      if (Found == CMap.end())
+        continue;
 
-      if (!Slot || Slot == GV)
+      GlobalVariable *Slot = Found->second;
+      if (Slot == GV)
         continue;
 
       if (!Slot->hasGlobalUnnamedAddr() && !GV->hasGlobalUnnamedAddr())
         continue;
 
       if (hasMetadataOtherThanDebugLoc(GV))
         continue;