cmd/link: retain only used interface methods

Currently, in the linker's deadcode pass, when an interface type
is live, the linker thinks all its methods are live, and uses
them to match methods on concrete types. The interface method may
never be used, though.

This CL changes it to only keep used interface methods, for
matching concrete type methods. To do that, when an interface
method is used, the compiler generates a mark relocation. The
linker uses the marker relocations to mark used interface
methods, and only the used ones.

binary size    before      after
cmd/compile   18887400   18812200
cmd/go        13470652   13470492

Change-Id: I3cfd9df4a53783330ba87735853f2a0ec3c42802
Reviewed-on: https://go-review.googlesource.com/c/go/+/256798
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

2 files changed, 40 insertions, 33 deletions

diff --git a/src/cmd/internal/objabi/reloctype.go b/src/cmd/internal/objabi/reloctype.go
index 1e328d659f..9e2e4a150a 100644
--- a/src/cmd/internal/objabi/reloctype.go
+++ b/src/cmd/internal/objabi/reloctype.go
@@ -94,6 +94,12 @@ const (
 	// This is a marker relocation (0-sized), for the linker's reachabililty
 	// analysis.
 	R_USEIFACE
+	// R_USEIFACEMETHOD marks an interface method that is used in the function
+	// this relocation is applied to. The target is an interface type descriptor.
+	// The addend is the offset of the method in the type descriptor.
+	// This is a marker relocation (0-sized), for the linker's reachabililty
+	// analysis.
+	R_USEIFACEMETHOD
 	// 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
diff --git a/src/cmd/internal/objabi/reloctype_string.go b/src/cmd/internal/objabi/reloctype_string.go
index caf24eea58..01df4cce62 100644
--- a/src/cmd/internal/objabi/reloctype_string.go
+++ b/src/cmd/internal/objabi/reloctype_string.go
@@ -33,42 +33,43 @@ func _() {
 	_ = x[R_USEFIELD-23]
 	_ = x[R_USETYPE-24]
 	_ = x[R_USEIFACE-25]
-	_ = x[R_METHODOFF-26]
-	_ = x[R_POWER_TOC-27]
-	_ = x[R_GOTPCREL-28]
-	_ = x[R_JMPMIPS-29]
-	_ = x[R_DWARFSECREF-30]
-	_ = x[R_DWARFFILEREF-31]
-	_ = x[R_ARM64_TLS_LE-32]
-	_ = x[R_ARM64_TLS_IE-33]
-	_ = x[R_ARM64_GOTPCREL-34]
-	_ = x[R_ARM64_GOT-35]
-	_ = x[R_ARM64_PCREL-36]
-	_ = x[R_ARM64_LDST8-37]
-	_ = x[R_ARM64_LDST32-38]
-	_ = x[R_ARM64_LDST64-39]
-	_ = x[R_ARM64_LDST128-40]
-	_ = x[R_POWER_TLS_LE-41]
-	_ = x[R_POWER_TLS_IE-42]
-	_ = x[R_POWER_TLS-43]
-	_ = x[R_ADDRPOWER_DS-44]
-	_ = x[R_ADDRPOWER_GOT-45]
-	_ = x[R_ADDRPOWER_PCREL-46]
-	_ = x[R_ADDRPOWER_TOCREL-47]
-	_ = x[R_ADDRPOWER_TOCREL_DS-48]
-	_ = x[R_RISCV_PCREL_ITYPE-49]
-	_ = x[R_RISCV_PCREL_STYPE-50]
-	_ = x[R_PCRELDBL-51]
-	_ = x[R_ADDRMIPSU-52]
-	_ = x[R_ADDRMIPSTLS-53]
-	_ = x[R_ADDRCUOFF-54]
-	_ = x[R_WASMIMPORT-55]
-	_ = x[R_XCOFFREF-56]
+	_ = x[R_USEIFACEMETHOD-26]
+	_ = x[R_METHODOFF-27]
+	_ = x[R_POWER_TOC-28]
+	_ = x[R_GOTPCREL-29]
+	_ = x[R_JMPMIPS-30]
+	_ = x[R_DWARFSECREF-31]
+	_ = x[R_DWARFFILEREF-32]
+	_ = x[R_ARM64_TLS_LE-33]
+	_ = x[R_ARM64_TLS_IE-34]
+	_ = x[R_ARM64_GOTPCREL-35]
+	_ = x[R_ARM64_GOT-36]
+	_ = x[R_ARM64_PCREL-37]
+	_ = x[R_ARM64_LDST8-38]
+	_ = x[R_ARM64_LDST32-39]
+	_ = x[R_ARM64_LDST64-40]
+	_ = x[R_ARM64_LDST128-41]
+	_ = x[R_POWER_TLS_LE-42]
+	_ = x[R_POWER_TLS_IE-43]
+	_ = x[R_POWER_TLS-44]
+	_ = x[R_ADDRPOWER_DS-45]
+	_ = x[R_ADDRPOWER_GOT-46]
+	_ = x[R_ADDRPOWER_PCREL-47]
+	_ = x[R_ADDRPOWER_TOCREL-48]
+	_ = x[R_ADDRPOWER_TOCREL_DS-49]
+	_ = x[R_RISCV_PCREL_ITYPE-50]
+	_ = x[R_RISCV_PCREL_STYPE-51]
+	_ = x[R_PCRELDBL-52]
+	_ = x[R_ADDRMIPSU-53]
+	_ = x[R_ADDRMIPSTLS-54]
+	_ = x[R_ADDRCUOFF-55]
+	_ = x[R_WASMIMPORT-56]
+	_ = x[R_XCOFFREF-57]
 }
 
-const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_WEAKADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CALLRISCVR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_METHODOFFR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
+const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_WEAKADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CALLRISCVR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_METHODOFFR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST32R_ARM64_LDST64R_ARM64_LDST128R_POWER_TLS_LER_POWER_TLS_IER_POWER_TLSR_ADDRPOWER_DSR_ADDRPOWER_GOTR_ADDRPOWER_PCRELR_ADDRPOWER_TOCRELR_ADDRPOWER_TOCREL_DSR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
 
-var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 60, 66, 72, 81, 92, 101, 112, 122, 133, 140, 147, 155, 163, 171, 177, 183, 189, 199, 208, 218, 229, 240, 250, 259, 272, 286, 300, 314, 330, 341, 354, 367, 381, 395, 410, 424, 438, 449, 463, 478, 495, 513, 534, 553, 572, 582, 593, 606, 617, 629, 639}
+var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 60, 66, 72, 81, 92, 101, 112, 122, 133, 140, 147, 155, 163, 171, 177, 183, 189, 199, 208, 218, 234, 245, 256, 266, 275, 288, 302, 316, 330, 346, 357, 370, 383, 397, 411, 426, 440, 454, 465, 479, 494, 511, 529, 550, 569, 588, 598, 609, 622, 633, 645, 655}
 
 func (i RelocType) String() string {
 	i -= 1