Add -Xfrontend -mergeable-traps as a way to emit mergeable traps

Author: kubamracek

include/swift/AST/IRGenOptions.h

@@ -478,6 +478,9 @@ class IRGenOptions {
   // Whether to run the HotColdSplitting pass when optimizing.
   unsigned EnableHotColdSplit : 1;
 
+  // Whether to emit mergeable or non-mergeable traps.
+  unsigned MergeableTraps : 1;
+
   /// The number of threads for multi-threaded code generation.
   unsigned NumThreads = 0;
 
@@ -567,7 +570,7 @@ class IRGenOptions {
         DisableReadonlyStaticObjects(false), CollocatedMetadataFunctions(false),
         ColocateTypeDescriptors(true), UseRelativeProtocolWitnessTables(false),
         UseFragileResilientProtocolWitnesses(false),
-        EnableHotColdSplit(false),
+        EnableHotColdSplit(false), MergeableTraps(false),
         CmdArgs(), SanitizeCoverage(llvm::SanitizerCoverageOptions()),
         TypeInfoFilter(TypeInfoDumpFilter::All),
         PlatformCCallingConvention(llvm::CallingConv::C), UseCASBackend(false),

include/swift/Option/FrontendOptions.td

@@ -1317,6 +1317,10 @@ def disable_split_cold_code :
   Flag<["-"], "disable-split-cold-code">,
   HelpText<"Disable splitting of cold code when optimizing">;
 
+def mergeable_traps :
+  Flag<["-"], "mergeable-traps">,
+  HelpText<"Emit mergeable traps even in optimized builds">;
+
 def enable_new_llvm_pass_manager :
   Flag<["-"], "enable-new-llvm-pass-manager">,
   HelpText<"Enable the new llvm pass manager">;

lib/Frontend/CompilerInvocation.cpp

@@ -3445,6 +3445,11 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     return true;
   }
 
+  Opts.MergeableTraps = Opts.shouldOptimize();
+  if (Args.hasArg(OPT_mergeable_traps)) {
+    Opts.MergeableTraps = true;
+  }
+
   Opts.EnableObjectiveCProtocolSymbolicReferences =
     Args.hasFlag(OPT_enable_objective_c_protocol_symbolic_references,
                  OPT_disable_objective_c_protocol_symbolic_references,

lib/IRGen/IRGenFunction.cpp

@@ -503,7 +503,7 @@ llvm::CallInst *IRBuilder::CreateNonMergeableTrap(IRGenModule &IGM,
     }
   }
 
-  if (IGM.IRGen.Opts.shouldOptimize()) {
+  if (!IGM.IRGen.Opts.MergeableTraps) {
     // Emit unique side-effecting inline asm calls in order to eliminate
     // the possibility that an LLVM optimization or code generation pass
     // will merge these blocks back together again. We emit an empty asm

test/embedded/traps-mergeable.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-emit-ir -enable-experimental-feature Extern -enable-experimental-feature Embedded -mergeable-traps -wmo -Xllvm -link-embedded-runtime=0 %s -O | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: optimized_stdlib
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+@_extern(c)
+public func external()
+
+public func test(i: Int, j: Int) {
+     precondition(i != 0, "precondition 1")
+     external()
+     precondition(j != 1, "precondition 2")
+}
+
+// CHECK-NOT: call void asm sideeffect ""
+
+// CHECK: define {{.*}}@"$s4main4test1i1jySi_SitF"
+// CHECK:   tail call void @llvm.trap()
+// CHECK:   unreachable
+// CHECK:   tail call void @llvm.trap()
+// CHECK:   unreachable
+// CHECK: }