[NewPM] Fix MergeFunctions scheduling

MergeFunctions (as well as HotColdSplitting an IROutliner) are
incorrectly scheduled under the new pass manager. The code makes
it look like they run towards the end of the module optimization
pipeline (as they should), while in reality the run at the start.
This is because the OptimizePM populated around them is only
scheduled later.

I'm fixing this by moving these three passes until after OptimizePM
to avoid splitting the function pass pipeline. It doesn't seem
important to me that some of the function passes run after these
late module passes.

Differential Revision: https://reviews.llvm.org/D115098

Author: nikic

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -1182,23 +1182,6 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
 
   addVectorPasses(Level, OptimizePM, /* IsFullLTO */ false);
 
-  // Split out cold code. Splitting is done late to avoid hiding context from
-  // other optimizations and inadvertently regressing performance. The tradeoff
-  // is that this has a higher code size cost than splitting early.
-  if (EnableHotColdSplit && !LTOPreLink)
-    MPM.addPass(HotColdSplittingPass());
-
-  // Search the code for similar regions of code. If enough similar regions can
-  // be found where extracting the regions into their own function will decrease
-  // the size of the program, we extract the regions, a deduplicate the
-  // structurally similar regions.
-  if (EnableIROutliner)
-    MPM.addPass(IROutlinerPass());
-
-  // Merge functions if requested.
-  if (PTO.MergeFunctions)
-    MPM.addPass(MergeFunctionsPass());
-
   // LoopSink pass sinks instructions hoisted by LICM, which serves as a
   // canonicalization pass that enables other optimizations. As a result,
   // LoopSink pass needs to be a very late IR pass to avoid undoing LICM
@@ -1226,6 +1209,23 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   for (auto &C : OptimizerLastEPCallbacks)
     C(MPM, Level);
 
+  // Split out cold code. Splitting is done late to avoid hiding context from
+  // other optimizations and inadvertently regressing performance. The tradeoff
+  // is that this has a higher code size cost than splitting early.
+  if (EnableHotColdSplit && !LTOPreLink)
+    MPM.addPass(HotColdSplittingPass());
+
+  // Search the code for similar regions of code. If enough similar regions can
+  // be found where extracting the regions into their own function will decrease
+  // the size of the program, we extract the regions, a deduplicate the
+  // structurally similar regions.
+  if (EnableIROutliner)
+    MPM.addPass(IROutlinerPass());
+
+  // Merge functions if requested.
+  if (PTO.MergeFunctions)
+    MPM.addPass(MergeFunctionsPass());
+
   if (PTO.CallGraphProfile)
     MPM.addPass(CGProfilePass());
 

llvm/test/Transforms/PhaseOrdering/X86/merge-functions.ll

@@ -90,15 +90,8 @@ bb3:                                              ; preds = %bb1, %bb2
 
 define i1 @test2(i32 %c) {
 ; CHECK-LABEL: @test2(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = add i32 [[C:%.*]], -100
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 20
-; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i32 [[SWITCH_TABLEIDX]] to i20
-; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i20 -490991, [[SWITCH_CAST]]
-; CHECK-NEXT:    [[TMP1:%.*]] = and i20 [[SWITCH_DOWNSHIFT]], 1
-; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = icmp ne i20 [[TMP1]], 0
-; CHECK-NEXT:    [[I_0:%.*]] = select i1 [[TMP0]], i1 [[SWITCH_MASKED]], i1 false
-; CHECK-NEXT:    ret i1 [[I_0]]
+; CHECK-NEXT:    [[TMP2:%.*]] = tail call i1 @test1(i32 [[TMP0:%.*]]) #[[ATTR0:[0-9]+]]
+; CHECK-NEXT:    ret i1 [[TMP2]]
 ;
 entry:
   %i = alloca i8, align 1