cmd/internal/obj/riscv,cmd/link: rework riscv64 call relocations

The riscv64 assembler and linker generate three types of calls.
Most calls are made via a single JAL instruction, however this is
limited to +/-1MB of text. In the case where a call target is
unreachable (or unknown), the JAL targets an AUIPC+JALR trampoline.
All other cases use AUIPC+JALR pairs, including the case where a
single function exceeds 1MB in text size, potentially making it
impossible to reach trampolines.

Currently, the single instruction JAL call is marked with R_RISCV_CALL
and the two instruction AUIPC+JALR call is marked with
R_RISCV_PCREL_ITYPE, which is also used for memory load instructions.
This means that we have no way to identify that the latter is a call.

Switch to using R_RISCV_CALL to mark the AUIPC+JALR pair (aligning
somewhat with the elf.R_RISCV_CALL, which is deprecated in favour of
elf.R_RISCV_CALL_PLT). Add R_RISCV_JAL and use this to mark the single
instruction JAL direct calls. This is clearer and allows us to map
elf.R_RISCV_CALL_PLT to Go's R_RISCV_CALL.

Add all three types to IsDirectCall, so that direct calls are correctly
identified when a function exceeds 1MB of text.

Fixes #62465

Change-Id: Id3eea09688a2b7d6e481eae9ed0aa0d1f9a3a48f
Reviewed-on: https://go-review.googlesource.com/c/go/+/520095
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Than McIntosh <thanm@google.com>

2 files changed, 16 insertions, 8 deletions

diff --git a/src/cmd/internal/obj/riscv/cpu.go b/src/cmd/internal/obj/riscv/cpu.go
index bfd5153da4..edd1ac820b 100644
--- a/src/cmd/internal/obj/riscv/cpu.go
+++ b/src/cmd/internal/obj/riscv/cpu.go
@@ -260,8 +260,13 @@ const (
 	// corresponding *obj.Prog uses the temporary register.
 	USES_REG_TMP = 1 << iota
 
-	// NEED_CALL_RELOC is set on JAL instructions to indicate that a
-	// R_RISCV_CALL relocation is needed.
+	// NEED_JAL_RELOC is set on JAL instructions to indicate that a
+	// R_RISCV_JAL relocation is needed.
+	NEED_JAL_RELOC
+
+	// NEED_CALL_RELOC is set on an AUIPC instruction to indicate that it
+	// is the first instruction in an AUIPC + JAL pair that needs a
+	// R_RISCV_CALL relocation.
 	NEED_CALL_RELOC
 
 	// NEED_PCREL_ITYPE_RELOC is set on AUIPC instructions to indicate that
diff --git a/src/cmd/internal/obj/riscv/obj.go b/src/cmd/internal/obj/riscv/obj.go
index 4df28a43f6..501d518019 100644
--- a/src/cmd/internal/obj/riscv/obj.go
+++ b/src/cmd/internal/obj/riscv/obj.go
@@ -41,7 +41,7 @@ func jalToSym(ctxt *obj.Link, p *obj.Prog, lr int16) {
 	}
 
 	p.As = AJAL
-	p.Mark |= NEED_CALL_RELOC
+	p.Mark |= NEED_JAL_RELOC
 	p.From.Type = obj.TYPE_REG
 	p.From.Reg = lr
 	p.Reg = obj.REG_NONE
@@ -610,7 +610,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	var callCount int
 	for p := cursym.Func().Text; p != nil; p = p.Link {
 		markRelocs(p)
-		if p.Mark&NEED_CALL_RELOC == NEED_CALL_RELOC {
+		if p.Mark&NEED_JAL_RELOC == NEED_JAL_RELOC {
 			callCount++
 		}
 	}
@@ -664,7 +664,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 					jmp.To = obj.Addr{Type: obj.TYPE_REG, Reg: REG_TMP}
 
 					p.As = AAUIPC
-					p.Mark = (p.Mark &^ NEED_CALL_RELOC) | NEED_PCREL_ITYPE_RELOC
+					p.Mark = (p.Mark &^ NEED_JAL_RELOC) | NEED_CALL_RELOC
 					p.AddRestSource(obj.Addr{Type: obj.TYPE_CONST, Offset: p.To.Offset, Sym: p.To.Sym})
 					p.From = obj.Addr{Type: obj.TYPE_CONST, Offset: 0}
 					p.Reg = obj.REG_NONE
@@ -2345,13 +2345,13 @@ func assemble(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	for p := cursym.Func().Text; p != nil; p = p.Link {
 		switch p.As {
 		case AJAL:
-			if p.Mark&NEED_CALL_RELOC == NEED_CALL_RELOC {
+			if p.Mark&NEED_JAL_RELOC == NEED_JAL_RELOC {
 				rel := obj.Addrel(cursym)
 				rel.Off = int32(p.Pc)
 				rel.Siz = 4
 				rel.Sym = p.To.Sym
 				rel.Add = p.To.Offset
-				rel.Type = objabi.R_RISCV_CALL
+				rel.Type = objabi.R_RISCV_JAL
 			}
 		case AJALR:
 			if p.To.Sym != nil {
@@ -2361,7 +2361,10 @@ func assemble(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 		case AAUIPC, AMOV, AMOVB, AMOVH, AMOVW, AMOVBU, AMOVHU, AMOVWU, AMOVF, AMOVD:
 			var addr *obj.Addr
 			var rt objabi.RelocType
-			if p.Mark&NEED_PCREL_ITYPE_RELOC == NEED_PCREL_ITYPE_RELOC {
+			if p.Mark&NEED_CALL_RELOC == NEED_CALL_RELOC {
+				rt = objabi.R_RISCV_CALL
+				addr = &p.From
+			} else if p.Mark&NEED_PCREL_ITYPE_RELOC == NEED_PCREL_ITYPE_RELOC {
 				rt = objabi.R_RISCV_PCREL_ITYPE
 				addr = &p.From
 			} else if p.Mark&NEED_PCREL_STYPE_RELOC == NEED_PCREL_STYPE_RELOC {