compiler, transform: remove runtime.isnil hack

This hack was originally introduced in
#251 to fix an escape analysis
regression after #222
introduced nil checks. Since a new optimization in LLVM (see
https://reviews.llvm.org/D60047) this hack is not necessary anymore and
can be removed.

I've compared all regular tests and smoke tests before and after to
check the size. In most cases this change was an improvement although
there are a few regressions.

Author: aykevl

compiler/asserts.go

@@ -176,23 +176,11 @@ func (b *builder) createNilCheck(inst ssa.Value, ptr llvm.Value, blockPrefix str
 	}
 
 	// Compare against nil.
-	var isnil llvm.Value
-	if ptr.Type().PointerAddressSpace() == 0 {
-		// Do the nil check using the isnil builtin, which marks the parameter
-		// as nocapture.
-		// The reason it has to go through a builtin, is that a regular icmp
-		// instruction may capture the pointer in LLVM semantics, see
-		// https://reviews.llvm.org/D60047 for details. Pointer capturing
-		// unfortunately breaks escape analysis, so we use this trick to let the
-		// functionattr pass know that this pointer doesn't really escape.
-		ptr = b.CreateBitCast(ptr, b.i8ptrType, "")
-		isnil = b.createRuntimeCall("isnil", []llvm.Value{ptr}, "")
-	} else {
-		// Do the nil check using a regular icmp. This can happen with function
-		// pointers on AVR, which don't benefit from escape analysis anyway.
-		nilptr := llvm.ConstPointerNull(ptr.Type())
-		isnil = b.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
-	}
+	// We previously used a hack to make sure this wouldn't break escape
+	// analysis, but this is not necessary anymore since
+	// https://reviews.llvm.org/D60047 has been merged.
+	nilptr := llvm.ConstPointerNull(ptr.Type())
+	isnil := b.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
 
 	// Emit the nil check in IR.
 	b.createRuntimeAssert(isnil, blockPrefix, "nilPanic")

compiler/compiler.go

@@ -339,9 +339,6 @@ func Compile(pkgName string, machine llvm.TargetMachine, config *compileopts.Con
 		c.mod.NamedFunction("runtime.alloc").AddAttributeAtIndex(0, getAttr(attrName))
 	}
 
-	// See createNilCheck in asserts.go.
-	c.mod.NamedFunction("runtime.isnil").AddAttributeAtIndex(1, nocapture)
-
 	// On *nix systems, the "abort" functuion in libc is used to handle fatal panics.
 	// Mark it as noreturn so LLVM can optimize away code.
 	if abort := c.mod.NamedFunction("abort"); !abort.IsNil() && abort.IsDeclaration() {

src/runtime/panic.go

@@ -27,14 +27,6 @@ func _recover() interface{} {
 	return nil
 }
 
-// See emitNilCheck in compiler/asserts.go.
-// This function is a dummy function that has its first and only parameter
-// marked 'nocapture' to work around a limitation in LLVM: a regular pointer
-// comparison captures the pointer.
-func isnil(ptr *uint8) bool {
-	return ptr == nil
-}
-
 // Panic when trying to dereference a nil pointer.
 func nilPanic() {
 	runtimePanic("nil pointer dereference")

transform/func-lowering.go

@@ -197,19 +197,8 @@ func LowerFuncValues(mod llvm.Module) {
 						panic("expected inttoptr")
 					}
 					for _, ptrUse := range getUses(callIntPtr) {
-						if !ptrUse.IsABitCastInst().IsNil() {
-							for _, bitcastUse := range getUses(ptrUse) {
-								if bitcastUse.IsACallInst().IsNil() || bitcastUse.CalledValue().IsAFunction().IsNil() {
-									panic("expected a call instruction")
-								}
-								switch bitcastUse.CalledValue().Name() {
-								case "runtime.isnil":
-									bitcastUse.ReplaceAllUsesWith(llvm.ConstInt(ctx.Int1Type(), 0, false))
-									bitcastUse.EraseFromParentAsInstruction()
-								default:
-									panic("expected a call to runtime.isnil")
-								}
-							}
+						if !ptrUse.IsAICmpInst().IsNil() {
+							ptrUse.ReplaceAllUsesWith(llvm.ConstInt(ctx.Int1Type(), 0, false))
 						} else if !ptrUse.IsACallInst().IsNil() && ptrUse.CalledValue() == callIntPtr {
 							addFuncLoweringSwitch(mod, builder, funcID, ptrUse, func(funcPtr llvm.Value, params []llvm.Value) llvm.Value {
 								return builder.CreateCall(funcPtr, params, "")

transform/optimizer.go

@@ -98,24 +98,6 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		OptimizeAllocs(mod)
 		OptimizeStringToBytes(mod)
 
-		// Lower runtime.isnil calls to regular nil comparisons.
-		isnil := mod.NamedFunction("runtime.isnil")
-		if !isnil.IsNil() {
-			builder := mod.Context().NewBuilder()
-			defer builder.Dispose()
-			for _, use := range getUses(isnil) {
-				builder.SetInsertPointBefore(use)
-				ptr := use.Operand(0)
-				if !ptr.IsABitCastInst().IsNil() {
-					ptr = ptr.Operand(0)
-				}
-				nilptr := llvm.ConstPointerNull(ptr.Type())
-				icmp := builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
-				use.ReplaceAllUsesWith(icmp)
-				use.EraseFromParentAsInstruction()
-			}
-		}
-
 	} else {
 		// Must be run at any optimization level.
 		err := LowerInterfaces(mod)

transform/testdata/func-lowering.ll

@@ -19,8 +19,6 @@ declare void @"internal/task.start"(i32, i8*, i8*, i8*)
 
 declare void @runtime.nilPanic(i8*, i8*)
 
-declare i1 @runtime.isnil(i8*, i8*, i8*)
-
 declare void @"main$1"(i32, i8*, i8*)
 
 declare void @"main$2"(i32, i8*, i8*)
@@ -38,9 +36,8 @@ define void @runFunc1(i8*, i32, i8, i8* %context, i8* %parentHandle) {
 entry:
   %3 = call i32 @runtime.getFuncPtr(i8* %0, i32 %1, %runtime.typecodeID* @"reflect/types.type:func:{basic:int8}{}", i8* undef, i8* null)
   %4 = inttoptr i32 %3 to void (i8, i8*, i8*)*
-  %5 = bitcast void (i8, i8*, i8*)* %4 to i8*
-  %6 = call i1 @runtime.isnil(i8* %5, i8* undef, i8* null)
-  br i1 %6, label %fpcall.nil, label %fpcall.next
+  %5 = icmp eq void (i8, i8*, i8*)* %4, null
+  br i1 %5, label %fpcall.nil, label %fpcall.next
 
 fpcall.nil:
   call void @runtime.nilPanic(i8* undef, i8* null)
@@ -58,9 +55,8 @@ define void @runFunc2(i8*, i32, i8, i8* %context, i8* %parentHandle) {
 entry:
   %3 = call i32 @runtime.getFuncPtr(i8* %0, i32 %1, %runtime.typecodeID* @"reflect/types.type:func:{basic:uint8}{}", i8* undef, i8* null)
   %4 = inttoptr i32 %3 to void (i8, i8*, i8*)*
-  %5 = bitcast void (i8, i8*, i8*)* %4 to i8*
-  %6 = call i1 @runtime.isnil(i8* %5, i8* undef, i8* null)
-  br i1 %6, label %fpcall.nil, label %fpcall.next
+  %5 = icmp eq void (i8, i8*, i8*)* %4, null
+  br i1 %5, label %fpcall.nil, label %fpcall.next
 
 fpcall.nil:
   call void @runtime.nilPanic(i8* undef, i8* null)

transform/testdata/func-lowering.out.ll

@@ -19,8 +19,6 @@ declare void @"internal/task.start"(i32, i8*, i8*, i8*)
 
 declare void @runtime.nilPanic(i8*, i8*)
 
-declare i1 @runtime.isnil(i8*, i8*, i8*)
-
 declare void @"main$1"(i32, i8*, i8*)
 
 declare void @"main$2"(i32, i8*, i8*)
@@ -38,9 +36,8 @@ define void @runFunc1(i8*, i32, i8, i8* %context, i8* %parentHandle) {
 entry:
   %3 = icmp eq i32 %1, 0
   %4 = select i1 %3, void (i8, i8*, i8*)* null, void (i8, i8*, i8*)* @funcInt8
-  %5 = bitcast void (i8, i8*, i8*)* %4 to i8*
-  %6 = call i1 @runtime.isnil(i8* %5, i8* undef, i8* null)
-  br i1 %6, label %fpcall.nil, label %fpcall.next
+  %5 = icmp eq void (i8, i8*, i8*)* %4, null
+  br i1 %5, label %fpcall.nil, label %fpcall.next
 
 fpcall.nil:
   call void @runtime.nilPanic(i8* undef, i8* null)