cmd/internal/objabi: rename R_USEGENERICIFACEMETHOD to R_USENAMEDMETHOD.

Now we have two cases when we want to keep methods with a specific name:
calls to generic interface methods and MethodByName("Foo"). Both use
the same relocation type, so let us give it a name that is not limited
to the implementation of generic interfaces.

Also, introduce staticdata.StrSymNoCommon(). It creates a symbol that
does not appear in the final binary and only communicates arguments
to the linker.

Change-Id: Icc9f49febfde1f31a4455b5acb903e8838d1c0af
Reviewed-on: https://go-review.googlesource.com/c/go/+/523016
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -18,6 +18,7 @@ import (
 	"cmd/compile/internal/compare"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/objw"
+	"cmd/compile/internal/staticdata"
 	"cmd/compile/internal/typebits"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
@@ -1848,17 +1849,8 @@ func MarkUsedIfaceMethod(n *ir.CallExpr) {
 		// some sort of fuzzy shape matching. For now, only use the name
 		// of the method for matching.
 		r := obj.Addrel(ir.CurFunc.LSym)
-		// We use a separate symbol just to tell the linker the method name.
-		// (The symbol itself is not needed in the final binary. Do not use
-		// staticdata.StringSym, which creates a content addessable symbol,
-		// which may have trailing zero bytes. This symbol doesn't need to
-		// be deduplicated anyway.)
-		name := dot.Sel.Name
-		var nameSym obj.LSym
-		nameSym.WriteString(base.Ctxt, 0, len(name), name)
-		objw.Global(&nameSym, int32(len(name)), obj.RODATA)
-		r.Sym = &nameSym
-		r.Type = objabi.R_USEGENERICIFACEMETHOD
+		r.Sym = staticdata.StringSymNoCommon(dot.Sel.Name)
+		r.Type = objabi.R_USENAMEDMETHOD
 		return
 	}
 

src/cmd/compile/internal/staticdata/data.go

@@ -96,6 +96,16 @@ func StringSym(pos src.XPos, s string) (data *obj.LSym) {
 	return symdata
 }
 
+// StringSymNoCommon is like StringSym, but produces a symbol that is not content-
+// addressable. This symbol is not supposed to appear in the final binary, it is
+// only used to pass string arguments to the linker like R_USENAMEDMETHOD does.
+func StringSymNoCommon(s string) (data *obj.LSym) {
+	var nameSym obj.LSym
+	nameSym.WriteString(base.Ctxt, 0, len(s), s)
+	objw.Global(&nameSym, int32(len(s)), obj.RODATA)
+	return &nameSym
+}
+
 // maxFileSize is the maximum file size permitted by the linker
 // (see issue #9862).
 const maxFileSize = int64(2e9)

src/cmd/compile/internal/walk/expr.go

@@ -12,7 +12,6 @@ import (
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
-	"cmd/compile/internal/objw"
 	"cmd/compile/internal/reflectdata"
 	"cmd/compile/internal/staticdata"
 	"cmd/compile/internal/typecheck"
@@ -1034,13 +1033,9 @@ func usemethod(n *ir.CallExpr) {
 	if ir.IsConst(targetName, constant.String) {
 		name := constant.StringVal(targetName.Val())
 
-		var nameSym obj.LSym
-		nameSym.WriteString(base.Ctxt, 0, len(name), name)
-		objw.Global(&nameSym, int32(len(name)), obj.RODATA)
-
 		r := obj.Addrel(ir.CurFunc.LSym)
-		r.Type = objabi.R_USEGENERICIFACEMETHOD
-		r.Sym = &nameSym
+		r.Type = objabi.R_USENAMEDMETHOD
+		r.Sym = staticdata.StringSymNoCommon(name)
 	} else {
 		ir.CurFunc.SetReflectMethod(true)
 		// The LSym is initialized at this point. We need to set the attribute on the LSym.

src/cmd/internal/objabi/reloctype.go

@@ -94,11 +94,10 @@ const (
 	// This is a marker relocation (0-sized), for the linker's reachabililty
 	// analysis.
 	R_USEIFACEMETHOD
-	// Similar to R_USEIFACEMETHOD, except instead of indicating a type +
-	// method offset with Sym+Add, Sym points to a symbol containing the name
-	// of the method being called. See the description in
-	// cmd/compile/internal/reflectdata/reflect.go:MarkUsedIfaceMethod for details.
-	R_USEGENERICIFACEMETHOD
+	// R_USENAMEDMETHOD marks that methods with a specific name must not be eliminated.
+	// The target is a symbol containing the name of a method called via a generic
+	// interface or looked up via MethodByName("F").
+	R_USENAMEDMETHOD
 	// R_METHODOFF resolves to a 32-bit offset from the beginning of the section
 	// holding the data being relocated to the referenced symbol.
 	// It is a variant of R_ADDROFF used when linking from the uncommonType of a

src/cmd/internal/objabi/reloctype_string.go

@@ -32,7 +32,7 @@ func _() {
 	_ = x[R_USETYPE-22]
 	_ = x[R_USEIFACE-23]
 	_ = x[R_USEIFACEMETHOD-24]
-	_ = x[R_USEGENERICIFACEMETHOD-25]
+	_ = x[R_USENAMEDMETHOD-25]
 	_ = x[R_METHODOFF-26]
 	_ = x[R_KEEP-27]
 	_ = x[R_POWER_TOC-28]
@@ -98,9 +98,9 @@ func _() {
 	_ = x[R_INITORDER-88]
 }
 
-const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_USEGENERICIFACEMETHODR_METHODOFFR_KEEPR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_PCREL_LDST8R_ARM64_PCREL_LDST16R_ARM64_PCREL_LDST32R_ARM64_PCREL_LDST64R_ARM64_LDST8R_ARM64_LDST16R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_POWER_TLS_IE_PCREL34R_POWER_TLS_LE_TPREL34R_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_GOT_PCREL34R_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_ADDRPOWER_D34R_ADDRPOWER_PCREL34R_RISCV_CALLR_RISCV_CALL_TRAMPR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_RISCV_TLS_IER_RISCV_TLS_LER_RISCV_GOT_HI20R_RISCV_PCREL_HI20R_RISCV_PCREL_LO12_IR_RISCV_PCREL_LO12_SR_RISCV_BRANCHR_RISCV_RVC_BRANCHR_RISCV_RVC_JUMPR_PCRELDBLR_ADDRLOONG64R_ADDRLOONG64UR_ADDRLOONG64TLSR_ADDRLOONG64TLSUR_CALLLOONG64R_LOONG64_TLS_IE_PCREL_HIR_LOONG64_TLS_IE_LOR_JMPLOONG64R_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREFR_PEIMAGEOFFR_INITORDER"
+const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_USENAMEDMETHODR_METHODOFFR_KEEPR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_PCREL_LDST8R_ARM64_PCREL_LDST16R_ARM64_PCREL_LDST32R_ARM64_PCREL_LDST64R_ARM64_LDST8R_ARM64_LDST16R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_POWER_TLS_IE_PCREL34R_POWER_TLS_LE_TPREL34R_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_GOT_PCREL34R_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_ADDRPOWER_D34R_ADDRPOWER_PCREL34R_RISCV_CALLR_RISCV_CALL_TRAMPR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_RISCV_TLS_IER_RISCV_TLS_LER_RISCV_GOT_HI20R_RISCV_PCREL_HI20R_RISCV_PCREL_LO12_IR_RISCV_PCREL_LO12_SR_RISCV_BRANCHR_RISCV_RVC_BRANCHR_RISCV_RVC_JUMPR_PCRELDBLR_ADDRLOONG64R_ADDRLOONG64UR_ADDRLOONG64TLSR_ADDRLOONG64TLSUR_CALLLOONG64R_LOONG64_TLS_IE_PCREL_HIR_LOONG64_TLS_IE_LOR_JMPLOONG64R_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREFR_PEIMAGEOFFR_INITORDER"
 
-var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 53, 59, 68, 79, 88, 99, 109, 116, 123, 131, 139, 147, 153, 159, 165, 175, 184, 194, 210, 233, 244, 250, 261, 271, 280, 293, 307, 321, 335, 351, 362, 375, 394, 414, 434, 454, 467, 481, 495, 509, 524, 538, 552, 563, 585, 607, 621, 636, 659, 676, 694, 715, 730, 749, 761, 779, 798, 817, 831, 845, 861, 879, 899, 919, 933, 951, 967, 977, 990, 1004, 1020, 1037, 1050, 1075, 1094, 1106, 1117, 1130, 1141, 1153, 1163, 1175, 1186}
+var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 53, 59, 68, 79, 88, 99, 109, 116, 123, 131, 139, 147, 153, 159, 165, 175, 184, 194, 210, 226, 237, 243, 254, 264, 273, 286, 300, 314, 328, 344, 355, 368, 387, 407, 427, 447, 460, 474, 488, 502, 517, 531, 545, 556, 578, 600, 614, 629, 652, 669, 687, 708, 723, 742, 754, 772, 791, 810, 824, 838, 854, 872, 892, 912, 926, 944, 960, 970, 983, 997, 1013, 1030, 1043, 1068, 1087, 1099, 1110, 1123, 1134, 1146, 1156, 1168, 1179}
 
 func (i RelocType) String() string {
 	i -= 1

src/cmd/link/internal/ld/deadcode.go

@@ -214,7 +214,7 @@ func (d *deadcodePass) flood() {
 				}
 				d.ifaceMethod[m] = true
 				continue
-			case objabi.R_USEGENERICIFACEMETHOD:
+			case objabi.R_USENAMEDMETHOD:
 				name := d.decodeGenericIfaceMethod(d.ldr, r.Sym())
 				if d.ctxt.Debugvlog > 1 {
 					d.ctxt.Logf("reached generic iface method: %s\n", name)