runtime: remove trailing empty arrays in structs

The ones at the end of M and G are just used to compute
their size for use in assembly.  Generate the size explicitly.
The one at the end of itab is variable-sized, and at least one.
The ones at the end of interfacetype and uncommontype are not
needed, as the preceding slice references them (the slice was
originally added for use by reflect?).
The one at the end of stackmap is already accessed correctly,
and the runtime never allocates one.

Update #9401

Change-Id: Ia75e3aaee38425f038c506868a17105bd64c712f
Reviewed-on: https://go-review.googlesource.com/2420Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/export.c

@@ -551,6 +551,7 @@ dumpasmhdr(void)
 			t = n->type;
 			if(t->etype != TSTRUCT || t->map != T || t->funarg)
 				break;
+			Bprint(b, "#define %s__size %d\n", t->sym->name, (int)t->width);
 			for(t=t->type; t != T; t=t->down)
 				if(!isblanksym(t->sym))
 					Bprint(b, "#define %s_%s %d\n", n->sym->name, t->sym->name, (int)t->width);

src/runtime/iface.go

@@ -35,8 +35,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 		if canfail {
 			return nil
 		}
-		i := (*imethod)(add(unsafe.Pointer(inter), unsafe.Sizeof(interfacetype{})))
-		panic(&TypeAssertionError{"", *typ._string, *inter.typ._string, *i.name})
+		panic(&TypeAssertionError{"", *typ._string, *inter.typ._string, *inter.mhdr[0].name})
 	}
 
 	// compiler has provided some good hash codes for us.
@@ -76,7 +75,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 		}
 	}
 
-	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr))*ptrSize, 0, &memstats.other_sys))
+	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*ptrSize, 0, &memstats.other_sys))
 	m.inter = inter
 	m._type = typ
 
@@ -89,15 +88,15 @@ search:
 	nt := len(x.mhdr)
 	j := 0
 	for k := 0; k < ni; k++ {
-		i := (*imethod)(add(unsafe.Pointer(inter), unsafe.Sizeof(interfacetype{})+uintptr(k)*unsafe.Sizeof(imethod{})))
+		i := &inter.mhdr[k]
 		iname := i.name
 		ipkgpath := i.pkgpath
 		itype := i._type
 		for ; j < nt; j++ {
-			t := (*method)(add(unsafe.Pointer(x), unsafe.Sizeof(uncommontype{})+uintptr(j)*unsafe.Sizeof(method{})))
+			t := &x.mhdr[j]
 			if t.mtyp == itype && t.name == iname && t.pkgpath == ipkgpath {
 				if m != nil {
-					*(*unsafe.Pointer)(add(unsafe.Pointer(m), unsafe.Sizeof(itab{})+uintptr(k)*ptrSize)) = t.ifn
+					*(*unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*ptrSize)) = t.ifn
 				}
 				goto nextimethod
 			}

src/runtime/malloc2.go

@@ -498,7 +498,7 @@ type bitvector struct {
 type stackmap struct {
 	n        int32   // number of bitmaps
 	nbit     int32   // number of bits in each bitmap
-	bytedata [0]byte // bitmaps, each starting on a 32-bit boundary
+	bytedata [1]byte // bitmaps, each starting on a 32-bit boundary
 }
 
 // Returns pointer map data for the given stackmap index

src/runtime/runtime2.go

@@ -215,7 +215,6 @@ type g struct {
 	gopc         uintptr // pc of go statement that created this goroutine
 	racectx      uintptr
 	waiting      *sudog // sudog structures this g is waiting on (that have a valid elem ptr)
-	end          [0]byte
 }
 
 type mts struct {
@@ -298,7 +297,6 @@ type m struct {
 	notesig *int8
 	errstr  *byte
 	//#endif
-	end [0]byte
 }
 
 type p struct {
@@ -425,7 +423,7 @@ type itab struct {
 	link   *itab
 	bad    int32
 	unused int32
-	fun    [0]uintptr
+	fun    [1]uintptr // variable sized
 }
 
 // Lock-free stack node.

src/runtime/sys_windows_386.s

@@ -189,22 +189,22 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	MOVL	SP, DX
 
 	// setup dummy m, g
-	SUBL	$m_end, SP		// space for M
+	SUBL	$m__size, SP		// space for M
 	MOVL	SP, 0(SP)
-	MOVL	$m_end, 4(SP)
+	MOVL	$m__size, 4(SP)
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
 
 	LEAL	m_tls(SP), CX
 	MOVL	CX, 0x14(FS)
 	MOVL	SP, BX
-	SUBL	$g_end, SP		// space for G
+	SUBL	$g__size, SP		// space for G
 	MOVL	SP, g(CX)
 	MOVL	SP, m_g0(BX)
 
 	MOVL	SP, 0(SP)
-	MOVL	$g_end, 4(SP)
+	MOVL	$g__size, 4(SP)
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
-	LEAL	g_end(SP), BX
+	LEAL	g__size(SP), BX
 	MOVL	BX, g_m(SP)
 	LEAL	-8192(SP), CX
 	MOVL	CX, (g_stack+stack_lo)(SP)

src/runtime/sys_windows_amd64.s

@@ -225,22 +225,22 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	MOVQ	SP, DX
 
 	// setup dummy m, g
-	SUBQ	$m_end, SP		// space for M
+	SUBQ	$m__size, SP		// space for M
 	MOVQ	SP, 0(SP)
-	MOVQ	$m_end, 8(SP)
+	MOVQ	$m__size, 8(SP)
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
 
 	LEAQ	m_tls(SP), CX
 	MOVQ	CX, 0x28(GS)
 	MOVQ	SP, BX
-	SUBQ	$g_end, SP		// space for G
+	SUBQ	$g__size, SP		// space for G
 	MOVQ	SP, g(CX)
 	MOVQ	SP, m_g0(BX)
 
 	MOVQ	SP, 0(SP)
-	MOVQ	$g_end, 8(SP)
+	MOVQ	$g__size, 8(SP)
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
-	LEAQ	g_end(SP), BX
+	LEAQ	g__size(SP), BX
 	MOVQ	BX, g_m(SP)
 
 	LEAQ	-8192(SP), CX

src/runtime/type.go

@@ -47,7 +47,6 @@ type uncommontype struct {
 	name    *string
 	pkgpath *string
 	mhdr    []method
-	m       [0]method
 }
 
 type imethod struct {
@@ -59,7 +58,6 @@ type imethod struct {
 type interfacetype struct {
 	typ  _type
 	mhdr []imethod
-	m    [0]imethod
 }
 
 type maptype struct {