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

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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.
This commit is contained in:
Jaden Weiss
2020-04-09 16:08:15 -04:00
committed by Ayke
parent 0a8bfc57ef
commit 9890c760cf
3 changed files with 110 additions and 121 deletions
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transform/func-lowering.go
+84 -96
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@@ -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()
}
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()
}
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
}, 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()
}
// 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()
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()
}
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()))
}
}
}