[DivRempairs] add a pass to optimize div/rem pairs (PR31028)

This is intended to be a superset of the functionality from D31037 (EarlyCSE) but implemented 
as an independent pass, so there's no stretching of scope and feature creep for an existing pass. 
I also proposed a weaker version of this for SimplifyCFG in D30910. And I initially had almost 
this same functionality as an addition to CGP in the motivating example of PR31028:
https://bugs.llvm.org/show_bug.cgi?id=31028

The advantage of positioning this ahead of SimplifyCFG in the pass pipeline is that it can allow 
more flattening. But it needs to be after passes (InstCombine) that could sink a div/rem and
undo the hoisting that is done here.

Decomposing remainder may allow removing some code from the backend (PPC and possibly others).

Differential Revision: https://reviews.llvm.org/D37121 

llvm-svn: 312862

2 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index 62e00baabaf..c277b5b14e7 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -92,6 +92,7 @@
 #include "llvm/Transforms/Scalar/CorrelatedValuePropagation.h"
 #include "llvm/Transforms/Scalar/DCE.h"
 #include "llvm/Transforms/Scalar/DeadStoreElimination.h"
+#include "llvm/Transforms/Scalar/DivRemPairs.h"
 #include "llvm/Transforms/Scalar/EarlyCSE.h"
 #include "llvm/Transforms/Scalar/Float2Int.h"
 #include "llvm/Transforms/Scalar/GVN.h"
@@ -765,6 +766,11 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   // And finally clean up LCSSA form before generating code.
   OptimizePM.addPass(InstSimplifierPass());
 
+  // This hoists/decomposes div/rem ops. It should run after other sink/hoist
+  // passes to avoid re-sinking, but before SimplifyCFG because it can allow
+  // flattening of blocks.
+  OptimizePM.addPass(DivRemPairsPass());
+
   // LoopSink (and other loop passes since the last simplifyCFG) might have
   // resulted in single-entry-single-exit or empty blocks. Clean up the CFG.
   OptimizePM.addPass(SimplifyCFGPass());
diff --git a/llvm/lib/Passes/PassRegistry.def b/llvm/lib/Passes/PassRegistry.def
index 5b8c3ada883..bfe3dd782c1 100644
--- a/llvm/lib/Passes/PassRegistry.def
+++ b/llvm/lib/Passes/PassRegistry.def
@@ -142,6 +142,7 @@ FUNCTION_PASS("break-crit-edges", BreakCriticalEdgesPass())
 FUNCTION_PASS("consthoist", ConstantHoistingPass())
 FUNCTION_PASS("correlated-propagation", CorrelatedValuePropagationPass())
 FUNCTION_PASS("dce", DCEPass())
+FUNCTION_PASS("div-rem-pairs", DivRemPairsPass())
 FUNCTION_PASS("dse", DSEPass())
 FUNCTION_PASS("dot-cfg", CFGPrinterPass())
 FUNCTION_PASS("dot-cfg-only", CFGOnlyPrinterPass())