cmd/internal/obj/riscv,cmd/link/internal/riscv64: add call trampolines for riscv64

CALL and JMP on riscv64 are currently implemented as an AUIPC+JALR pair. This means
that every call requires two instructions and makes use of the REG_TMP register,
even when the symbol would be directly reachable via a single JAL instruction.

Add support for call trampolines - CALL and JMP are now implemented as a single JAL
instruction, with the linker generating trampolines in the case where the symbol is
not reachable (more than +/-1MiB from the JAL instruction), is an unknown symbol or
does not yet have an address assigned. Each trampoline contains an AUIPC+JALR pair,
which the relocation is applied to.

Due to the limited reachability of the JAL instruction, combined with the way that
the Go linker currently assigns symbol addresses, there are cases where a call is to
a symbol that has no address currently assigned. In this situation we have to assume
that a trampoline will be required, however we can patch this up during relocation,
potentially calling directly instead. This means that we will end up with trampolines
that are unused. In the case of the Go binary, there are around 3,500 trampolines of
which approximately 2,300 are unused (around 9200 bytes of machine instructions).

Overall, this removes over 72,000 AUIPC instructions from the Go binary.

Change-Id: I2d9ecfb85dfc285c7729a3cd0b3a77b6f6c98be0
Reviewed-on: https://go-review.googlesource.com/c/go/+/345051
Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

2 files changed, 61 insertions, 53 deletions

diff --git a/src/cmd/internal/objabi/reloctype.go b/src/cmd/internal/objabi/reloctype.go
index 52827a6dee..0cc60fbe3b 100644
--- a/src/cmd/internal/objabi/reloctype.go
+++ b/src/cmd/internal/objabi/reloctype.go
@@ -59,8 +59,6 @@ const (
 	// R_CALLMIPS (only used on mips64) resolves to non-PC-relative target address
 	// of a CALL (JAL) instruction, by encoding the address into the instruction.
 	R_CALLMIPS
-	// R_CALLRISCV marks RISC-V CALLs for stack checking.
-	R_CALLRISCV
 	R_CONST
 	R_PCREL
 	// R_TLS_LE, used on 386, amd64, and ARM, resolves to the offset of the
@@ -218,6 +216,15 @@ const (
 
 	// RISC-V.
 
+	// R_RISCV_CALL relocates a J-type instruction with a 21 bit PC-relative
+	// address.
+	R_RISCV_CALL
+
+	// R_RISCV_CALL_TRAMP is the same as R_RISCV_CALL but denotes the use of a
+	// trampoline, which we may be able to avoid during relocation. These are
+	// only used by the linker and are not emitted by the compiler or assembler.
+	R_RISCV_CALL_TRAMP
+
 	// R_RISCV_PCREL_ITYPE resolves a 32-bit PC-relative address using an
 	// AUIPC + I-type instruction pair.
 	R_RISCV_PCREL_ITYPE
@@ -274,7 +281,7 @@ const (
 // the target address in register or memory.
 func (r RelocType) IsDirectCall() bool {
 	switch r {
-	case R_CALL, R_CALLARM, R_CALLARM64, R_CALLMIPS, R_CALLPOWER, R_CALLRISCV:
+	case R_CALL, R_CALLARM, R_CALLARM64, R_CALLMIPS, R_CALLPOWER, R_RISCV_CALL, R_RISCV_CALL_TRAMP:
 		return true
 	}
 	return false
diff --git a/src/cmd/internal/objabi/reloctype_string.go b/src/cmd/internal/objabi/reloctype_string.go
index 4638ef14d9..f2e06a5b21 100644
--- a/src/cmd/internal/objabi/reloctype_string.go
+++ b/src/cmd/internal/objabi/reloctype_string.go
@@ -20,59 +20,60 @@ func _() {
 	_ = x[R_CALLIND-10]
 	_ = x[R_CALLPOWER-11]
 	_ = x[R_CALLMIPS-12]
-	_ = x[R_CALLRISCV-13]
-	_ = x[R_CONST-14]
-	_ = x[R_PCREL-15]
-	_ = x[R_TLS_LE-16]
-	_ = x[R_TLS_IE-17]
-	_ = x[R_GOTOFF-18]
-	_ = x[R_PLT0-19]
-	_ = x[R_PLT1-20]
-	_ = x[R_PLT2-21]
-	_ = x[R_USEFIELD-22]
-	_ = x[R_USETYPE-23]
-	_ = x[R_USEIFACE-24]
-	_ = x[R_USEIFACEMETHOD-25]
-	_ = x[R_METHODOFF-26]
-	_ = x[R_KEEP-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_LDST16-39]
-	_ = x[R_ARM64_LDST32-40]
-	_ = x[R_ARM64_LDST64-41]
-	_ = x[R_ARM64_LDST128-42]
-	_ = x[R_POWER_TLS_LE-43]
-	_ = x[R_POWER_TLS_IE-44]
-	_ = x[R_POWER_TLS-45]
-	_ = x[R_ADDRPOWER_DS-46]
-	_ = x[R_ADDRPOWER_GOT-47]
-	_ = x[R_ADDRPOWER_PCREL-48]
-	_ = x[R_ADDRPOWER_TOCREL-49]
-	_ = x[R_ADDRPOWER_TOCREL_DS-50]
-	_ = x[R_RISCV_PCREL_ITYPE-51]
-	_ = x[R_RISCV_PCREL_STYPE-52]
-	_ = x[R_RISCV_TLS_IE_ITYPE-53]
-	_ = x[R_RISCV_TLS_IE_STYPE-54]
-	_ = x[R_PCRELDBL-55]
-	_ = x[R_ADDRMIPSU-56]
-	_ = x[R_ADDRMIPSTLS-57]
-	_ = x[R_ADDRCUOFF-58]
-	_ = x[R_WASMIMPORT-59]
-	_ = x[R_XCOFFREF-60]
+	_ = x[R_CONST-13]
+	_ = x[R_PCREL-14]
+	_ = x[R_TLS_LE-15]
+	_ = x[R_TLS_IE-16]
+	_ = x[R_GOTOFF-17]
+	_ = x[R_PLT0-18]
+	_ = x[R_PLT1-19]
+	_ = x[R_PLT2-20]
+	_ = x[R_USEFIELD-21]
+	_ = x[R_USETYPE-22]
+	_ = x[R_USEIFACE-23]
+	_ = x[R_USEIFACEMETHOD-24]
+	_ = x[R_METHODOFF-25]
+	_ = x[R_KEEP-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_LDST16-38]
+	_ = 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_CALL-50]
+	_ = x[R_RISCV_CALL_TRAMP-51]
+	_ = x[R_RISCV_PCREL_ITYPE-52]
+	_ = x[R_RISCV_PCREL_STYPE-53]
+	_ = x[R_RISCV_TLS_IE_ITYPE-54]
+	_ = x[R_RISCV_TLS_IE_STYPE-55]
+	_ = x[R_PCRELDBL-56]
+	_ = x[R_ADDRMIPSU-57]
+	_ = x[R_ADDRMIPSTLS-58]
+	_ = x[R_ADDRCUOFF-59]
+	_ = x[R_WASMIMPORT-60]
+	_ = x[R_XCOFFREF-61]
 }
 
-const _RelocType_name = "R_ADDRR_ADDRPOWERR_ADDRARM64R_ADDRMIPSR_ADDROFFR_SIZER_CALLR_CALLARMR_CALLARM64R_CALLINDR_CALLPOWERR_CALLMIPSR_CALLRISCVR_CONSTR_PCRELR_TLS_LER_TLS_IER_GOTOFFR_PLT0R_PLT1R_PLT2R_USEFIELDR_USETYPER_USEIFACER_USEIFACEMETHODR_METHODOFFR_KEEPR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST16R_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_RISCV_TLS_IE_ITYPER_RISCV_TLS_IE_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
+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_METHODOFFR_KEEPR_POWER_TOCR_GOTPCRELR_JMPMIPSR_DWARFSECREFR_DWARFFILEREFR_ARM64_TLS_LER_ARM64_TLS_IER_ARM64_GOTPCRELR_ARM64_GOTR_ARM64_PCRELR_ARM64_LDST8R_ARM64_LDST16R_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_CALLR_RISCV_CALL_TRAMPR_RISCV_PCREL_ITYPER_RISCV_PCREL_STYPER_RISCV_TLS_IE_ITYPER_RISCV_TLS_IE_STYPER_PCRELDBLR_ADDRMIPSUR_ADDRMIPSTLSR_ADDRCUOFFR_WASMIMPORTR_XCOFFREF"
 
-var _RelocType_index = [...]uint16{0, 6, 17, 28, 38, 47, 53, 59, 68, 79, 88, 99, 109, 120, 127, 134, 142, 150, 158, 164, 170, 176, 186, 195, 205, 221, 232, 238, 249, 259, 268, 281, 295, 309, 323, 339, 350, 363, 376, 390, 404, 418, 433, 447, 461, 472, 486, 501, 518, 536, 557, 576, 595, 615, 635, 645, 656, 669, 680, 692, 702}
+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, 221, 227, 238, 248, 257, 270, 284, 298, 312, 328, 339, 352, 365, 379, 393, 407, 422, 436, 450, 461, 475, 490, 507, 525, 546, 558, 576, 595, 614, 634, 654, 664, 675, 688, 699, 711, 721}
 
 func (i RelocType) String() string {
 	i -= 1