cmd/internal/obj/riscv: add environment call/breakpoint and base counter/timer instructions

This implements assembler support for ECALL/EBREAK, along with base
counter/timer instructions.

Based on riscv-go port.

Updates #27532

Change-Id: I690a9fd835eeddee1fe9a5616d2b2f856d3952b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/195918Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/riscvenc.s

@@ -116,6 +116,19 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	REMW	X5, X6, X7				// bb635302
 	REMUW	X5, X6, X7				// bb735302
 
+	// 10.1: Base Counters and Timers
+	RDCYCLE		X5				// f32200c0
+	RDTIME		X5				// f32210c0
+	RDINSTRET	X5				// f32220c0
+
+	// Privileged ISA
+
+	// 3.2.1: Environment Call and Breakpoint
+	ECALL						// 73000000
+	SCALL						// 73000000
+	EBREAK						// 73001000
+	SBREAK						// 73001000
+
 	// Arbitrary bytes (entered in little-endian mode)
 	WORD	$0x12345678	// WORD $305419896	// 78563412
 	WORD	$0x9abcdef0	// WORD $2596069104	// f0debc9a

src/cmd/internal/obj/riscv/obj.go

@@ -68,6 +68,35 @@ func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 			p.As = ASRAI
 		}
 	}
+
+	switch p.As {
+	case obj.AUNDEF, AECALL, AEBREAK, ASCALL, ASBREAK, ARDCYCLE, ARDTIME, ARDINSTRET:
+		switch p.As {
+		case obj.AUNDEF:
+			p.As = AEBREAK
+		case ASCALL:
+			// SCALL is the old name for ECALL.
+			p.As = AECALL
+		case ASBREAK:
+			// SBREAK is the old name for EBREAK.
+			p.As = AEBREAK
+		}
+
+		ins := encode(p.As)
+		if ins == nil {
+			panic("progedit: tried to rewrite nonexistent instruction")
+		}
+
+		// The CSR isn't exactly an offset, but it winds up in the
+		// immediate area of the encoded instruction, so record it in
+		// the Offset field.
+		p.From.Type = obj.TYPE_CONST
+		p.From.Offset = ins.csr
+		p.Reg = REG_ZERO
+		if p.To.Type == obj.TYPE_NONE {
+			p.To.Type, p.To.Reg = obj.TYPE_REG, REG_ZERO
+		}
+	}
 }
 
 // setPCs sets the Pc field in all instructions reachable from p.
@@ -391,6 +420,17 @@ var encodingForAs = [ALAST & obj.AMask]encoding{
 	AREMW & obj.AMask:   rIIIEncoding,
 	AREMUW & obj.AMask:  rIIIEncoding,
 
+	// 10.1: Base Counters and Timers
+	ARDCYCLE & obj.AMask:   iIEncoding,
+	ARDTIME & obj.AMask:    iIEncoding,
+	ARDINSTRET & obj.AMask: iIEncoding,
+
+	// Privileged ISA
+
+	// 3.2.1: Environment Call and Breakpoint
+	AECALL & obj.AMask:  iIEncoding,
+	AEBREAK & obj.AMask: iIEncoding,
+
 	// Escape hatch
 	AWORD & obj.AMask: rawEncoding,