compiler: move ReplacePanicsWithTrap pass to transforms

This moves the transformation pass to the right location, and adds tests
to see that it actually works correctly.

transform/panic.go

@@ -0,0 +1,33 @@
+package transform
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// ReplacePanicsWithTrap replaces each call to panic (or similar functions) with
+// calls to llvm.trap, to reduce code size. This is the -panic=trap command-line
+// option.
+func ReplacePanicsWithTrap(mod llvm.Module) {
+	ctx := mod.Context()
+	builder := ctx.NewBuilder()
+	defer builder.Dispose()
+
+	trap := mod.NamedFunction("llvm.trap")
+	if trap.IsNil() {
+		trapType := llvm.FunctionType(ctx.VoidType(), nil, false)
+		trap = llvm.AddFunction(mod, "llvm.trap", trapType)
+	}
+	for _, name := range []string{"runtime._panic", "runtime.runtimePanic"} {
+		fn := mod.NamedFunction(name)
+		if fn.IsNil() {
+			continue
+		}
+		for _, use := range getUses(fn) {
+			if use.IsACallInst().IsNil() || use.CalledValue() != fn {
+				panic("expected use of a panic function to be a call")
+			}
+			builder.SetInsertPointBefore(use)
+			builder.CreateCall(trap, nil, "")
+		}
+	}
+}

transform/panic_test.go

@@ -0,0 +1,10 @@
+package transform
+
+import (
+	"testing"
+)
+
+func TestReplacePanicsWithTrap(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/panic", ReplacePanicsWithTrap)
+}

transform/testdata/panic.ll

@@ -0,0 +1,22 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@"runtime.lookupPanic$string" = constant [18 x i8] c"index out of range"
+
+declare void @runtime.runtimePanic(i8*, i32)
+
+declare void @runtime._panic(i32, i8*)
+
+define void @runtime.lookupPanic() {
+  call void @runtime.runtimePanic(i8* getelementptr inbounds ([18 x i8], [18 x i8]* @"runtime.lookupPanic$string", i64 0, i64 0), i32 18)
+  ret void
+}
+
+; This is equivalent to the following code:
+;     func someFunc(x interface{}) {
+;         panic(x)
+;     }
+define void @someFunc(i32 %typecode, i8* %value) {
+  call void @runtime._panic(i32 %typecode, i8* %value)
+  unreachable
+}

transform/testdata/panic.out.ll

@@ -0,0 +1,25 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@"runtime.lookupPanic$string" = constant [18 x i8] c"index out of range"
+
+declare void @runtime.runtimePanic(i8*, i32)
+
+declare void @runtime._panic(i32, i8*)
+
+define void @runtime.lookupPanic() {
+  call void @llvm.trap()
+  call void @runtime.runtimePanic(i8* getelementptr inbounds ([18 x i8], [18 x i8]* @"runtime.lookupPanic$string", i64 0, i64 0), i32 18)
+  ret void
+}
+
+define void @someFunc(i32 %typecode, i8* %value) {
+  call void @llvm.trap()
+  call void @runtime._panic(i32 %typecode, i8* %value)
+  unreachable
+}
+
+; Function Attrs: cold noreturn nounwind
+declare void @llvm.trap() #0
+
+attributes #0 = { cold noreturn nounwind }