Disable partial inlining by default and add a flag for it. (#4191)

Locally I saw a 10% speedup on j2cl but reports of regressions have
arrived, so let's disable it for now pending investigation. The option added
here should make it easy to experiment.

3 files changed, 16 insertions, 2 deletions

diff --git a/src/pass.h b/src/pass.h
index 802a34ab5..2bb34aeea 100644
--- a/src/pass.h
+++ b/src/pass.h
@@ -82,6 +82,11 @@ struct InliningOptions {
   // Loops usually mean the function does heavy work, so the call overhead
   // is not significant and we do not inline such functions by default.
   bool allowFunctionsWithLoops = false;
+  // The number of ifs to allow partial inlining of their conditions. A value of
+  // zero disables partial inlining.
+  // TODO: Investigate enabling this. Locally 4 appears useful on real-world
+  //       code, but reports of regressions have arrived.
+  Index partialInliningIfs = 0;
 };
 
 struct PassOptions {
diff --git a/src/passes/Inlining.cpp b/src/passes/Inlining.cpp
index e1ae4c2a8..7467d2559 100644
--- a/src/passes/Inlining.cpp
+++ b/src/passes/Inlining.cpp
@@ -641,8 +641,7 @@ private:
     // without an else.
 
     // Find the number of ifs.
-    // TODO: Investigate more values here. 4 appears useful on real-world code.
-    const Index MaxIfs = 4;
+    const Index MaxIfs = options.inlining.partialInliningIfs;
     Index numIfs = 0;
     while (getIf(body, numIfs) && numIfs <= MaxIfs) {
       numIfs++;
diff --git a/src/tools/optimization-options.h b/src/tools/optimization-options.h
index 58a8e3980..8d526f92f 100644
--- a/src/tools/optimization-options.h
+++ b/src/tools/optimization-options.h
@@ -171,6 +171,16 @@ struct OptimizationOptions : public ToolOptions {
            [this](Options* o, const std::string&) {
              passOptions.inlining.allowFunctionsWithLoops = true;
            })
+      .add("--partial-inlining-ifs",
+           "-pii",
+           "Number of ifs allowed in partial inlining (zero means partial "
+           "inlining is disabled) (default: " +
+             std::to_string(InliningOptions().partialInliningIfs) + ')',
+           Options::Arguments::One,
+           [this](Options* o, const std::string& argument) {
+             passOptions.inlining.partialInliningIfs =
+               static_cast<Index>(std::stoi(argument));
+           })
       .add("--ignore-implicit-traps",
            "-iit",
            "Optimize under the helpful assumption that no surprising traps "