transform (func-lowering): remove specializations from function value lowering and fix lowering of a function value of an unimplemented type

niaow · niaow · commit 4f068787d212 · 2020-04-09T20:21:32.000-04:00
Previously, the function value lowering pass had special cases for when there were 0 or 1 function implementations.
However, the results of the pass were incorrect in both of these cases.
This change removes the specializations and fixes the transformation.

In the case that there was a single function implementation, the compiler emitted a select instruction to obtain the function pointer.
This selected between null and the implementing function pointer.
While this was technically correct, it failed to eliminate indirect function calls.
This prevented discovery of these calls by the coroutine lowering pass, and caused async function calls to be passed through unlowered.
As a result, the generated code had undefined behavior (usually resulting in a segfault).

In the case of no function implementations, the lowering code was correct.
However, the lowering code was not run.
The discovery of function signatures was accomplished by scanning implementations, and when there were no implementations nothing was discovered or lowered.

For maintainability reasons, I have removed both specializations rather than fixing them.
This substantially simplifies the code, and reduces the amount of variation that we need to worry about for testing purposes.
The IR now generated in the cases of 0 or 1 function implementations can be efficiently simplified by LLVM's optimization passes.
Therefore, there should not be a substantial regression in terms of performance or machine code size.
diff --git a/transform/func-lowering.go b/transform/func-lowering.go
@@ -6,6 +6,7 @@ package transform
 import (
 	"sort"
 	"strconv"
+	"strings"
 
 	"github.com/tinygo-org/tinygo/compiler/llvmutil"
 	"tinygo.org/x/go-llvm"
@@ -55,17 +56,30 @@ func LowerFuncValues(mod llvm.Module) {
 	funcValueWithSignaturePtr := llvm.PointerType(mod.GetTypeByName("runtime.funcValueWithSignature"), 0)
 	signatures := map[string]*funcSignatureInfo{}
 	for global := mod.FirstGlobal(); !global.IsNil(); global = llvm.NextGlobal(global) {
-		if global.Type() != funcValueWithSignaturePtr {
+		var sig, funcVal llvm.Value
+		switch {
+		case global.Type() == funcValueWithSignaturePtr:
+			sig = llvm.ConstExtractValue(global.Initializer(), []uint32{1})
+			funcVal = global
+		case strings.HasPrefix(global.Name(), "reflect/types.type:func:{"):
+			sig = global
+		default:
 			continue
 		}
-		sig := llvm.ConstExtractValue(global.Initializer(), []uint32{1})
+
 		name := sig.Name()
+		var funcValueWithSignatures []llvm.Value
+		if funcVal.IsNil() {
+			funcValueWithSignatures = []llvm.Value{}
+		} else {
+			funcValueWithSignatures = []llvm.Value{funcVal}
+		}
 		if info, ok := signatures[name]; ok {
-			info.funcValueWithSignatures = append(info.funcValueWithSignatures, global)
+			info.funcValueWithSignatures = append(info.funcValueWithSignatures, funcValueWithSignatures...)
 		} else {
 			signatures[name] = &funcSignatureInfo{
 				sig:                     sig,
-				funcValueWithSignatures: []llvm.Value{global},
+				funcValueWithSignatures: funcValueWithSignatures,
 			}
 		}
 	}
@@ -123,95 +137,64 @@ func LowerFuncValues(mod llvm.Module) {
 				panic("expected all call uses to be runtime.getFuncPtr")
 			}
 			funcID := getFuncPtrCall.Operand(1)
-			switch len(functions) {
-			case 0:
-				// There are no functions used in a func value that implement
-				// this signature. The only possible value is a nil value.
-				for _, inttoptr := range getUses(getFuncPtrCall) {
-					if inttoptr.IsAIntToPtrInst().IsNil() {
-						panic("expected inttoptr")
-					}
-					nilptr := llvm.ConstPointerNull(inttoptr.Type())
-					inttoptr.ReplaceAllUsesWith(nilptr)
-					inttoptr.EraseFromParentAsInstruction()
+
+			// There are functions used in a func value that
+			// implement this signature.
+			// What we'll do is transform the following:
+			//     rawPtr := runtime.getFuncPtr(func.ptr)
+			//     if rawPtr == nil {
+			//         runtime.nilPanic()
+			//     }
+			//     result := rawPtr(...args, func.context)
+			// into this:
+			//     if false {
+			//         runtime.nilPanic()
+			//     }
+			//     var result // Phi
+			//     switch fn.id {
+			//     case 0:
+			//         runtime.nilPanic()
+			//     case 1:
+			//         result = call first implementation...
+			//     case 2:
+			//         result = call second implementation...
+			//     default:
+			//         unreachable
+			//     }
+
+			// Remove some casts, checks, and the old call which we're going
+			// to replace.
+			for _, callIntPtr := range getUses(getFuncPtrCall) {
+				if !callIntPtr.IsACallInst().IsNil() && callIntPtr.CalledValue().Name() == "internal/task.start" {
+					// Special case for goroutine starts.
+					addFuncLoweringSwitch(mod, builder, funcID, callIntPtr, func(funcPtr llvm.Value, params []llvm.Value) llvm.Value {
+						i8ptrType := llvm.PointerType(ctx.Int8Type(), 0)
+						calleeValue := builder.CreatePtrToInt(funcPtr, uintptrType, "")
+						start := mod.NamedFunction("internal/task.start")
+						builder.CreateCall(start, []llvm.Value{calleeValue, callIntPtr.Operand(1), llvm.Undef(i8ptrType), llvm.ConstNull(i8ptrType)}, "")
+						return llvm.Value{} // void so no return value
+					}, functions)
+					callIntPtr.EraseFromParentAsInstruction()
+					continue
 				}
-				getFuncPtrCall.EraseFromParentAsInstruction()
-			case 1:
-				// There is exactly one function with this signature that is
-				// used in a func value. The func value itself can be either nil
-				// or this one function.
-				builder.SetInsertPointBefore(getFuncPtrCall)
-				zero := llvm.ConstInt(uintptrType, 0, false)
-				isnil := builder.CreateICmp(llvm.IntEQ, funcID, zero, "")
-				funcPtrNil := llvm.ConstPointerNull(functions[0].funcPtr.Type())
-				funcPtr := builder.CreateSelect(isnil, funcPtrNil, functions[0].funcPtr, "")
-				for _, inttoptr := range getUses(getFuncPtrCall) {
-					if inttoptr.IsAIntToPtrInst().IsNil() {
-						panic("expected inttoptr")
-					}
-					inttoptr.ReplaceAllUsesWith(funcPtr)
-					inttoptr.EraseFromParentAsInstruction()
+				if callIntPtr.IsAIntToPtrInst().IsNil() {
+					panic("expected inttoptr")
 				}
-				getFuncPtrCall.EraseFromParentAsInstruction()
-			default:
-				// There are multiple functions used in a func value that
-				// implement this signature.
-				// What we'll do is transform the following:
-				//     rawPtr := runtime.getFuncPtr(func.ptr)
-				//     if rawPtr == nil {
-				//         runtime.nilPanic()
-				//     }
-				//     result := rawPtr(...args, func.context)
-				// into this:
-				//     if false {
-				//         runtime.nilPanic()
-				//     }
-				//     var result // Phi
-				//     switch fn.id {
-				//     case 0:
-				//         runtime.nilPanic()
-				//     case 1:
-				//         result = call first implementation...
-				//     case 2:
-				//         result = call second implementation...
-				//     default:
-				//         unreachable
-				//     }
-
-				// Remove some casts, checks, and the old call which we're going
-				// to replace.
-				for _, callIntPtr := range getUses(getFuncPtrCall) {
-					if !callIntPtr.IsACallInst().IsNil() && callIntPtr.CalledValue().Name() == "internal/task.start" {
-						// Special case for goroutine starts.
-						addFuncLoweringSwitch(mod, builder, funcID, callIntPtr, func(funcPtr llvm.Value, params []llvm.Value) llvm.Value {
-							i8ptrType := llvm.PointerType(ctx.Int8Type(), 0)
-							calleeValue := builder.CreatePtrToInt(funcPtr, uintptrType, "")
-							start := mod.NamedFunction("internal/task.start")
-							builder.CreateCall(start, []llvm.Value{calleeValue, callIntPtr.Operand(1), llvm.Undef(i8ptrType), llvm.ConstNull(i8ptrType)}, "")
-							return llvm.Value{} // void so no return value
+				for _, ptrUse := range getUses(callIntPtr) {
+					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, "")
 						}, functions)
-						callIntPtr.EraseFromParentAsInstruction()
-						continue
-					}
-					if callIntPtr.IsAIntToPtrInst().IsNil() {
-						panic("expected inttoptr")
-					}
-					for _, ptrUse := range getUses(callIntPtr) {
-						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, "")
-							}, functions)
-						} else {
-							panic("unexpected getFuncPtrCall")
-						}
-						ptrUse.EraseFromParentAsInstruction()
+					} else {
+						panic("unexpected getFuncPtrCall")
 					}
-					callIntPtr.EraseFromParentAsInstruction()
+					ptrUse.EraseFromParentAsInstruction()
 				}
-				getFuncPtrCall.EraseFromParentAsInstruction()
+				callIntPtr.EraseFromParentAsInstruction()
 			}
+			getFuncPtrCall.EraseFromParentAsInstruction()
 		}
 	}
 }
@@ -270,13 +253,18 @@ func addFuncLoweringSwitch(mod llvm.Module, builder llvm.Builder, funcID, call l
 		phiBlocks[i] = bb
 		phiValues[i] = result
 	}
-	// Create the PHI node so that the call result flows into the
-	// next block (after the split). This is only necessary when the
-	// call produced a value.
 	if call.Type().TypeKind() != llvm.VoidTypeKind {
-		builder.SetInsertPointBefore(nextBlock.FirstInstruction())
-		phi := builder.CreatePHI(call.Type(), "")
-		phi.AddIncoming(phiValues, phiBlocks)
-		call.ReplaceAllUsesWith(phi)
+		if len(functions) > 0 {
+			// Create the PHI node so that the call result flows into the
+			// next block (after the split). This is only necessary when the
+			// call produced a value.
+			builder.SetInsertPointBefore(nextBlock.FirstInstruction())
+			phi := builder.CreatePHI(call.Type(), "")
+			phi.AddIncoming(phiValues, phiBlocks)
+			call.ReplaceAllUsesWith(phi)
+		} else {
+			// This is always a nil panic, so replace the call result with undef.
+			call.ReplaceAllUsesWith(llvm.Undef(call.Type()))
+		}
 	}
 }
diff --git a/transform/testdata/func-lowering.ll b/transform/testdata/func-lowering.ll
@@ -4,10 +4,9 @@ target triple = "wasm32-unknown-unknown-wasm"
 %runtime.typecodeID = type { %runtime.typecodeID*, i32 }
 %runtime.funcValueWithSignature = type { i32, %runtime.typecodeID* }
 
-@"reflect/types.type:func:{basic:int8}{}" = external constant %runtime.typecodeID
 @"reflect/types.type:func:{basic:uint8}{}" = external constant %runtime.typecodeID
 @"reflect/types.type:func:{basic:int}{}" = external constant %runtime.typecodeID
-@"funcInt8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @funcInt8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:int8}{}" }
+@"reflect/types.type:func:{}{basic:uint32}" = external constant %runtime.typecodeID
 @"func1Uint8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @func1Uint8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:uint8}{}" }
 @"func2Uint8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @func2Uint8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:uint8}{}" }
 @"main$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i32, i8*, i8*)* @"main$1" to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:int}{}" }
@@ -23,29 +22,26 @@ declare void @"main$1"(i32, i8*, i8*)
 
 declare void @"main$2"(i32, i8*, i8*)
 
-declare void @funcInt8(i8, i8*, i8*)
-
 declare void @func1Uint8(i8, i8*, i8*)
 
 declare void @func2Uint8(i8, i8*, i8*)
 
-; Call a function of which only one function with this signature is used as a
-; function value. This means that lowering it to IR is trivial: simply check
-; whether the func value is nil, and if not, call that one function directly.
-define void @runFunc1(i8*, i32, i8, i8* %context, i8* %parentHandle) {
+; There are no functions with this signature used in a func value.
+; This means that this should unconditionally nil panic.
+define i32 @runFuncNone(i8*, i32, 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 = icmp eq void (i8, i8*, i8*)* %4, null
-  br i1 %5, label %fpcall.nil, label %fpcall.next
+  %2 = call i32 @runtime.getFuncPtr(i8* %0, i32 %1, %runtime.typecodeID* @"reflect/types.type:func:{}{basic:uint32}", i8* undef, i8* null)
+  %3 = inttoptr i32 %2 to i32 (i8*, i8*)*
+  %4 = icmp eq i32 (i8*, i8*)* %3, null
+  br i1 %4, label %fpcall.nil, label %fpcall.next
 
 fpcall.nil:
   call void @runtime.nilPanic(i8* undef, i8* null)
   unreachable
 
 fpcall.next:
-  call void %4(i8 %2, i8* %0, i8* undef)
-  ret void
+  %5 = call i32 %3(i8* %0, i8* undef)
+  ret i32 %5
 }
 
 ; There are two functions with this signature used in a func value. That means
diff --git a/transform/testdata/func-lowering.out.ll b/transform/testdata/func-lowering.out.ll
@@ -4,10 +4,9 @@ target triple = "wasm32-unknown-unknown-wasm"
 %runtime.typecodeID = type { %runtime.typecodeID*, i32 }
 %runtime.funcValueWithSignature = type { i32, %runtime.typecodeID* }
 
-@"reflect/types.type:func:{basic:int8}{}" = external constant %runtime.typecodeID
 @"reflect/types.type:func:{basic:uint8}{}" = external constant %runtime.typecodeID
 @"reflect/types.type:func:{basic:int}{}" = external constant %runtime.typecodeID
-@"funcInt8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @funcInt8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:int8}{}" }
+@"reflect/types.type:func:{}{basic:uint32}" = external constant %runtime.typecodeID
 @"func1Uint8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @func1Uint8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:uint8}{}" }
 @"func2Uint8$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i8, i8*, i8*)* @func2Uint8 to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:uint8}{}" }
 @"main$withSignature" = constant %runtime.funcValueWithSignature { i32 ptrtoint (void (i32, i8*, i8*)* @"main$1" to i32), %runtime.typecodeID* @"reflect/types.type:func:{basic:int}{}" }
@@ -23,29 +22,34 @@ declare void @"main$1"(i32, i8*, i8*)
 
 declare void @"main$2"(i32, i8*, i8*)
 
-declare void @funcInt8(i8, i8*, i8*)
-
 declare void @func1Uint8(i8, i8*, i8*)
 
 declare void @func2Uint8(i8, i8*, i8*)
 
-; Call a function of which only one function with this signature is used as a
-; function value. This means that lowering it to IR is trivial: simply check
-; whether the func value is nil, and if not, call that one function directly.
-define void @runFunc1(i8*, i32, i8, i8* %context, i8* %parentHandle) {
+; This function calls a function pointer argument with no available implementations.
+; It will unconditionally run into a nil panic.
+define i32 @runFuncNone(i8*, i32, 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 = icmp eq void (i8, i8*, i8*)* %4, null
-  br i1 %5, label %fpcall.nil, label %fpcall.next
+  br i1 false, label %fpcall.nil, label %fpcall.next
 
 fpcall.nil:
   call void @runtime.nilPanic(i8* undef, i8* null)
   unreachable
 
 fpcall.next:
-  call void %4(i8 %2, i8* %0, i8* undef)
-  ret void
+  switch i32 %1, label %func.default [
+    i32 0, label %func.nil
+  ]
+
+func.nil:
+  call void @runtime.nilPanic(i8* undef, i8* null)
+  unreachable
+
+func.next:
+  ret i32 undef
+
+func.default:
+  unreachable
 }
 
 ; There are two functions with this signature used in a func value. That means
