[FuncSpec] Respect MaxConstantsThreshold

[SjoerdMeijer]

Summary

This is a follow up of D115458 and splits off the function change.

This truncates the worklist to 'MaxConstantsThreshold' candidates if the MaxConstantsThreshold was exceeded. It changes the behaviour of option -func-specialization-max-constants. Before it didn't specialise at all when this threshold was exceeded, but now it specialises up to MaxConstantsThreshold candidates from the sorted worklist.

[snehasish]

lgtm.

FYI in case you were not aware - if you have a dependent patch you can "Edit Related Revision > Edit Parent Revision" to ensure that the CI picks up the parent patch.

[SjoerdMeijer]

In D115509#3195111, @snehasish wrote:

lgtm.

FYI in case you were not aware - if you have a dependent patch you can "Edit Related Revision > Edit Parent Revision" to ensure that the CI picks up the parent patch.

Ok, thanks! Also for the reviews of this patch and the other.

Just to let you know my plans in case you're interested. I am going to pick this up further and will be working on function specialisation some more. I am going to add some more tests, will work on some of the limitations, look at compile times which are actually good when it triggers but it's a bit too high when it doesn't, and start looking in supporting this with ThinLTO. So yes, to be continued. :-)

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

for (Argument &FormalArg : F->args()) {
 
       if (Worklist.empty())
         continue;
 
       // Sort the candidates in descending order.
       llvm::sort(Worklist,
                  [] (ArgInfo &L, ArgInfo &R) { return L.Gain > R.Gain; });
 
-      // TODO: truncate the worklist to 'MaxConstantsThreshold' candidates if
+      // Truncate the worklist to 'MaxConstantsThreshold' candidates if
       // necessary.
       if (Worklist.size() > MaxConstantsThreshold) {
-        Worklist.clear();
-        continue;
+        LLVM_DEBUG(dbgs() << "FnSpecialization: number of constants exceed "
+                    << "the maximum number of constants threshold.\n"
+                    << "Truncating worklist to " << MaxConstantsThreshold
+                    << " candidates.\n");
+        Worklist.erase(Worklist.begin() + MaxConstantsThreshold,
+                       Worklist.end());
       }
 
       if (IsPartial || Worklist.size() < ActualConstArg.size())
         for (auto &ActualArg : Worklist)
           ActualArg.Partial = true;
 
       LLVM_DEBUG(dbgs() << "Sorted list of candidates by gain:\n";
                  for (auto &C

llvm/test/Transforms/FunctionSpecialization/function-specialization4.ll

 entry:
   %0 = load i32, i32* %b, align 4
   %add = add nsw i32 %x, %0
   %1 = load i32, i32* %c, align 4
   %add1 = add nsw i32 %add, %1
   ret i32 %add1
 }
 
-; CONST1-NOT: define internal i32 @foo.1(i32 %x, i32* %b, i32* %c)
+; CONST1:     define internal i32 @foo.1(i32 %x, i32* %b, i32* %c)
 ; CONST1-NOT: define internal i32 @foo.2(i32 %x, i32* %b, i32* %c)
 
 ; CHECK:        define internal i32 @foo.1(i32 %x, i32* %b, i32* %c) {
 ; CHECK-NEXT:   entry:
 ; CHECK-NEXT:     %0 = load i32, i32* @A, align 4
 ; CHECK-NEXT:     %add = add nsw i32 %x, %0
 ; CHECK-NEXT:     %1 = load i32, i32* %c, align 4
 ; CHECK-NEXT:     %add1 = add nsw i32 %add, %1