ColdSplit: avoid with -Osize

Based on prior evaluation, this optimization always increases code
size. It splits out blocks and introduces calls, which always adds
extra instructions to shuffle values for calling conventions and
to make the actual call/return. Apple clang avoids the optimization
in `-Oz`, which I think is pretty close to what Swift's `-Osize`
really means.

Author: kavon

lib/IRGen/IRGen.cpp

@@ -222,7 +222,8 @@ void swift::performLLVMOptimizations(const IRGenOptions &Opts,
     PTO.LoopVectorization = true;
     PTO.SLPVectorization = true;
     PTO.MergeFunctions = true;
-    DoHotColdSplit = Opts.EnableHotColdSplit;
+    // Splitting trades code size to enhance memory locality, avoid in -Osize.
+    DoHotColdSplit = Opts.EnableHotColdSplit && !Opts.optimizeForSize();
     level = llvm::OptimizationLevel::Os;
   } else {
     level = llvm::OptimizationLevel::O0;

test/IRGen/cold_split.swift

@@ -7,6 +7,11 @@
 // RUN:   -enable-throws-prediction -O -disable-split-cold-code \
 // RUN:       | %FileCheck --check-prefix CHECK-DISABLED %s
 
+//// Test using -Osize doesn't yield a split.
+// RUN: %target-swift-frontend %s -module-name=test -emit-assembly \
+// RUN:   -enable-throws-prediction -Osize -enable-split-cold-code \
+// RUN:       | %FileCheck --check-prefix CHECK-DISABLED %s
+
 //// Test disabling optimization entirely doesn't yield a split.
 // RUN: %target-swift-frontend %s -module-name=test -emit-assembly \
 // RUN:   -enable-throws-prediction -enable-split-cold-code \