cmd/link: consider interface conversions only in reachable code

The linker prunes methods that are not directly reachable if the
receiver type is never converted to interface. Currently, this
"never" is too strong: it is invalidated even if the interface
conversion is in an unreachable function. This CL improves it by
only considering interface conversions in reachable code. To do
that, we introduce a marker relocation R_USEIFACE, which marks
the target symbol as UsedInIface if the source symbol is reached.

binary size    before      after
cmd/compile   18897528   18887400
cmd/go        13607372   13470652

Change-Id: I66c6b69eeff9ae02d84d2e6f2bc7f1b29dd53910
Reviewed-on: https://go-review.googlesource.com/c/go/+/256797
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

2 files changed, 69 insertions, 2 deletions

diff --git a/src/cmd/internal/objabi/reloctype.go b/src/cmd/internal/objabi/reloctype.go
index f029a3c396..1e328d659f 100644
--- a/src/cmd/internal/objabi/reloctype.go
+++ b/src/cmd/internal/objabi/reloctype.go
@@ -89,6 +89,11 @@ const (
 	// should be linked into the final binary, even if there are no other
 	// direct references. (This is used for types reachable by reflection.)
 	R_USETYPE
+	// R_USEIFACE marks a type is converted to an interface in the function this
+	// relocation is applied to. The target is a type descriptor.
+	// This is a marker relocation (0-sized), for the linker's reachabililty
+	// analysis.
+	R_USEIFACE
 	// 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 83dfe71e07..caf24eea58 100644
--- a/src/cmd/internal/objabi/reloctype_string.go
+++ b/src/cmd/internal/objabi/reloctype_string.go
@@ -4,9 +4,71 @@ package objabi
 
 import "strconv"
 
-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_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"
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[R_ADDR-1]
+	_ = x[R_ADDRPOWER-2]
+	_ = x[R_ADDRARM64-3]
+	_ = x[R_ADDRMIPS-4]
+	_ = x[R_ADDROFF-5]
+	_ = x[R_WEAKADDROFF-6]
+	_ = x[R_SIZE-7]
+	_ = x[R_CALL-8]
+	_ = x[R_CALLARM-9]
+	_ = x[R_CALLARM64-10]
+	_ = x[R_CALLIND-11]
+	_ = x[R_CALLPOWER-12]
+	_ = x[R_CALLMIPS-13]
+	_ = x[R_CALLRISCV-14]
+	_ = x[R_CONST-15]
+	_ = x[R_PCREL-16]
+	_ = x[R_TLS_LE-17]
+	_ = x[R_TLS_IE-18]
+	_ = x[R_GOTOFF-19]
+	_ = x[R_PLT0-20]
+	_ = x[R_PLT1-21]
+	_ = x[R_PLT2-22]
+	_ = 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]
+}
+
+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"
 
-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, 219, 230, 240, 249, 262, 276, 290, 304, 320, 331, 344, 357, 371, 385, 400, 414, 428, 439, 453, 468, 485, 503, 524, 543, 562, 572, 583, 596, 607, 619, 629}
+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}
 
 func (i RelocType) String() string {
 	i -= 1