[WebAssembly] Disable register coalescing at -O1

[aheejin]

Summary

This disables RegisterCoalescer pass at -O1, which currently runs for
all levels except for -O0, as a part of common optimization pipeline.

RegisterCoalescer pass degrades Wasm debug info quality by a
significant margin. When I use LiveDebugValue analysis, disabling this
increases the average PC ranges covered by 15% on Emscripten core
benchmarks (52% -> 66.8%). (Our code is currently not using
LiveDebugValues analysis at the moment, and the experiment was done on
a local setting that enabled it. I'm planning to upstream it soon.)

In Emscripten core benchmarks, disabling this at -O1 causes +4.5% in
code size and +1% in the number of locals. The number of globals stays
the same. I believe this tradeoff is acceptable given that -O1 is not
usually used in production builds and is often used for debugging when
the application size is very large.

The plan is to investigate and fix what's causing the degradation in
that pass, but for now disabling it seems like a low-hanging quick fix.

[dschuff]

LGTM! nice investigation too.

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

WebAssemblyTargetMachine &getWebAssemblyTargetMachine() const {
     return getTM<WebAssemblyTargetMachine>();
   }
 
   FunctionPass *createTargetRegisterAllocator(bool) override;
 
   void addIRPasses() override;
   void addISelPrepare() override;
   bool addInstSelector() override;
+  void addOptimizedRegAlloc() override;
   void addPostRegAlloc() override;
   bool addGCPasses() override { return false; }
   void addPreEmitPass() override;
   bool addPreISel() override;
 
   // No reg alloc
   bool addRegAssignAndRewriteFast() override { return false; }
 
bool WebAssemblyPassConfig::addInstSelector() {
   addPass(createWebAssemblySetP2AlignOperands());
 
   // Eliminate range checks and add default targets to br_table instructions.
   addPass(createWebAssemblyFixBrTableDefaults());
 
   return false;
 }
 
+void WebAssemblyPassConfig::addOptimizedRegAlloc() {
+  // Currently RegisterCoalesce degrades wasm debug info quality by a
+  // significant margin. As a quick fix, disable this for -O1, which is often
+  // used for debugging large applications. Disabling this increases code size
+  // of Emscripten core benchmarks by ~5%, which is acceptable for -O1, which is
+  // usually not used for production builds.
+  // TODO Investigate why RegisterCoalesce degrades debug info quality and fix
+  // it properly
+  if (getOptLevel() == CodeGenOpt::Less)
+    disablePass(&RegisterCoalescerID);
+  TargetPassConfig::addOptimizedRegAlloc();
+}
+
 void WebAssemblyPassConfig::addPostRegAlloc() {
   // TODO: The following CodeGen passes don't currently support code containing
   // virtual registers. Consider removing their restrictions and re-enabling
   // them.
 
   // These functions all require the NoVRegs property.
   disablePass(&MachineCopyPropagationID);
   disablePass(&PostRAMachineSinkingID);

llvm/test/CodeGen/WebAssembly/regcoalesce-disable.ll

+; RUN: llc < %s -O1 --debug-pass=Structure 2>&1 | FileCheck %s --check-prefix=O1
+; RUN: llc < %s -O2 --debug-pass=Structure 2>&1 | FileCheck %s --check-prefix=O2
+
+; Test if RegisterCoalesce pass is disabled in -O1.
+
+; O1-NOT: Simple Register Coalescing
+; O2: Simple Register Coalescing
+target triple = "wasm32-unknown-unknown"
+
+define void @test() {
+  ret void
+}