cmd/compile,runtime: dedup writeBarrier needed

The writeBarrier needed struct member has the exact same value as
enabled, and used interchangeably.

I'm not sure if we plan to make a distinction between the two, but today
they are effectively the same, so dedup it and rely only on enabled.

src/cmd/compile/internal/typecheck/_builtin/runtime.go

@@ -160,7 +160,6 @@ func closechan(hchan any)
 var writeBarrier struct {
 	enabled bool
 	pad     [3]byte
-	needed  bool
 	cgo     bool
 	alignme uint64
 }

src/cmd/compile/internal/typecheck/builtin.go

@@ -340,7 +340,7 @@ func runtimeTypes() []*types.Type {
 	typs[102] = types.NewChan(typs[2], types.Csend)
 	typs[103] = newSig(params(typs[102], typs[3]), nil)
 	typs[104] = types.NewArray(typs[0], 3)
-	typs[105] = types.NewStruct([]*types.Field{types.NewField(src.NoXPos, Lookup("enabled"), typs[6]), types.NewField(src.NoXPos, Lookup("pad"), typs[104]), types.NewField(src.NoXPos, Lookup("needed"), typs[6]), types.NewField(src.NoXPos, Lookup("cgo"), typs[6]), types.NewField(src.NoXPos, Lookup("alignme"), typs[24])})
+	typs[105] = types.NewStruct([]*types.Field{types.NewField(src.NoXPos, Lookup("enabled"), typs[6]), types.NewField(src.NoXPos, Lookup("pad"), typs[104]), types.NewField(src.NoXPos, Lookup("cgo"), typs[6]), types.NewField(src.NoXPos, Lookup("alignme"), typs[24])})
 	typs[106] = newSig(params(typs[1], typs[3], typs[3]), nil)
 	typs[107] = newSig(params(typs[1], typs[3]), nil)
 	typs[108] = newSig(params(typs[1], typs[3], typs[15], typs[3], typs[15]), params(typs[15]))

src/runtime/mbarrier.go

@@ -153,7 +153,7 @@ func typedmemmove(typ *abi.Type, dst, src unsafe.Pointer) {
 	if dst == src {
 		return
 	}
-	if writeBarrier.needed && typ.PtrBytes != 0 {
+	if writeBarrier.enabled && typ.PtrBytes != 0 {
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes)
 	}
 	// There's a race here: if some other goroutine can write to
@@ -222,7 +222,7 @@ func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
 //
 //go:nosplit
 func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr, regs *abi.RegArgs) {
-	if writeBarrier.needed && typ != nil && typ.PtrBytes != 0 && size >= goarch.PtrSize {
+	if writeBarrier.enabled && typ != nil && typ.PtrBytes != 0 && size >= goarch.PtrSize {
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
 	}
 	memmove(dst, src, size)
@@ -277,7 +277,7 @@ func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe
 	// and growslice and reflect_typedslicecopy check for pointers
 	// before calling typedslicecopy.
 	size := uintptr(n) * typ.Size_
-	if writeBarrier.needed {
+	if writeBarrier.enabled {
 		pwsize := size - typ.Size_ + typ.PtrBytes
 		bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize)
 	}
@@ -307,7 +307,7 @@ func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
 //
 //go:nosplit
 func typedmemclr(typ *_type, ptr unsafe.Pointer) {
-	if writeBarrier.needed && typ.PtrBytes != 0 {
+	if writeBarrier.enabled && typ.PtrBytes != 0 {
 		bulkBarrierPreWrite(uintptr(ptr), 0, typ.PtrBytes)
 	}
 	memclrNoHeapPointers(ptr, typ.Size_)
@@ -320,7 +320,7 @@ func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
 
 //go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
 func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
-	if writeBarrier.needed && typ.PtrBytes != 0 {
+	if writeBarrier.enabled && typ.PtrBytes != 0 {
 		bulkBarrierPreWrite(uintptr(ptr), 0, size)
 	}
 	memclrNoHeapPointers(ptr, size)
@@ -329,7 +329,7 @@ func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintpt
 //go:linkname reflect_typedarrayclear reflect.typedarrayclear
 func reflect_typedarrayclear(typ *_type, ptr unsafe.Pointer, len int) {
 	size := typ.Size_ * uintptr(len)
-	if writeBarrier.needed && typ.PtrBytes != 0 {
+	if writeBarrier.enabled && typ.PtrBytes != 0 {
 		bulkBarrierPreWrite(uintptr(ptr), 0, size)
 	}
 	memclrNoHeapPointers(ptr, size)

src/runtime/mbitmap.go

@@ -563,7 +563,7 @@ func bulkBarrierPreWrite(dst, src, size uintptr) {
 	if (dst|src|size)&(goarch.PtrSize-1) != 0 {
 		throw("bulkBarrierPreWrite: unaligned arguments")
 	}
-	if !writeBarrier.needed {
+	if !writeBarrier.enabled {
 		return
 	}
 	if s := spanOf(dst); s == nil {
@@ -633,7 +633,7 @@ func bulkBarrierPreWriteSrcOnly(dst, src, size uintptr) {
 	if (dst|src|size)&(goarch.PtrSize-1) != 0 {
 		throw("bulkBarrierPreWrite: unaligned arguments")
 	}
-	if !writeBarrier.needed {
+	if !writeBarrier.enabled {
 		return
 	}
 	buf := &getg().m.p.ptr().wbBuf
@@ -718,7 +718,7 @@ func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
 		println("runtime: typeBitsBulkBarrier with type ", toRType(typ).string(), " with GC prog")
 		throw("runtime: invalid typeBitsBulkBarrier")
 	}
-	if !writeBarrier.needed {
+	if !writeBarrier.enabled {
 		return
 	}
 	ptrmask := typ.GCData

src/runtime/mgc.go

@@ -218,7 +218,6 @@ var gcphase uint32
 var writeBarrier struct {
 	enabled bool    // compiler emits a check of this before calling write barrier
 	pad     [3]byte // compiler uses 32-bit load for "enabled" field
-	needed  bool    // identical to enabled, for now (TODO: dedup)
 	alignme uint64  // guarantee alignment so that compiler can use a 32 or 64-bit load
 }
 
@@ -236,8 +235,7 @@ const (
 //go:nosplit
 func setGCPhase(x uint32) {
 	atomic.Store(&gcphase, x)
-	writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
-	writeBarrier.enabled = writeBarrier.needed
+	writeBarrier.enabled = gcphase == _GCmark || gcphase == _GCmarktermination
 }
 
 // gcMarkWorkerMode represents the mode that a concurrent mark worker

src/runtime/mgcmark.go

@@ -1064,7 +1064,7 @@ func gcDrainMarkWorkerFractional(gcw *gcWork) {
 //
 //go:nowritebarrier
 func gcDrain(gcw *gcWork, flags gcDrainFlags) {
-	if !writeBarrier.needed {
+	if !writeBarrier.enabled {
 		throw("gcDrain phase incorrect")
 	}
 
@@ -1178,7 +1178,7 @@ done:
 //go:nowritebarrier
 //go:systemstack
 func gcDrainN(gcw *gcWork, scanWork int64) int64 {
-	if !writeBarrier.needed {
+	if !writeBarrier.enabled {
 		throw("gcDrainN phase incorrect")
 	}