fix(asm): match RISC-V branch and jump encodings

CHANGELOG.md

@@ -75,6 +75,13 @@ Unreleased changes on the `development` branch.
   `ADDI`/`LUI`/`ADDIW` to `C.LI`/`C.LUI`/`C.ADDIW` when their immediate
   fits six signed bits.  A byte-exact ground-truth test covers zero, small,
   negative, and 32-bit immediates against `GOARCH=riscv64 go tool asm`.
+- **RISC-V branch/jump compression.**  `JMP`/`JAL` were being compressed to
+  `C.J` and `BEQ`/`BNE` (with `X0`) to `C.BEQZ`/`C.BNEZ`, but `go tool asm`
+  never emits these compressed forms.  They now emit the 32-bit `JAL` and
+  branch encodings the toolchain writes; the dead `C.J`/`C.BEQZ`/`C.BNEZ`
+  encoders were removed, and the `C.LUI` direct-instruction compression now
+  uses the correct six-bit signed range.  A byte-exact ground-truth test
+  covers the branch family and jumps against `GOARCH=riscv64 go tool asm`.
 - **Debugger watchpoint slots.**  `gasm debug`'s `watch` command always used
   hardware watchpoint slot 0, so a second `watch` call silently overwrote
   the first.  Watchpoint slots are now tracked in the `Session` (DR0–DR3);

asm/riscv_assemble.go

@@ -203,7 +203,8 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		word = riscvIType(riscvEnc{0x13, 0x0, 0x00}, 0, 0, 0)
 		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
 	case "JMP":
-		// JMP = JAL X0, target.  Try C.J compression.
+		// JMP = JAL X0, target.  The Go assembler never compresses this to
+		// C.J, so always emit the 32-bit JAL.
 		var target string
 		if len(ops) >= 1 {
 			target = labelFromOperand(ops[0])
@@ -213,11 +214,6 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 			return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 		}
 		offset := int32(targetOff - pc)
-		// C.J: funct3=0x5, offset in ±2 KB, bit 0 must be 0.
-		if offset >= -2048 && offset <= 2046 && offset%2 == 0 {
-			c16 := rvcCJ(0x5, offset)
-			return []byte{byte(c16), byte(c16 >> 8)}, nil
-		}
 		word = riscvJType(0, offset)
 		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
 	case "JAL":
@@ -234,11 +230,6 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 			return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 		}
 		offset := int32(targetOff - pc)
-		// JAL X0, target → C.J when offset fits.
-		if rd == 0 && offset >= -2048 && offset <= 2046 && offset%2 == 0 {
-			c16 := rvcCJ(0x5, offset)
-			return []byte{byte(c16), byte(c16 >> 8)}, nil
-		}
 		word = riscvJType(rd, offset)
 		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
 
@@ -492,18 +483,7 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 			return nil, fmt.Errorf("invalid register in %s", mnem)
 		}
 
-		// Try C.BEQZ / C.BNEZ compression.
+		// The Go assembler never compresses branches to C.BEQZ/C.BNEZ.
-		if (mnem == "BEQ" || mnem == "BNE") && rs2 == 0 && isRVCIntReg(rs1) {
-			if cOff := offset; cOff >= -256 && cOff <= 254 && cOff%2 == 0 {
-				funct3 := uint32(0x6) // C.BEQZ
-				if mnem == "BNE" {
-					funct3 = 0x7 // C.BNEZ
-				}
-				c16 := rvcCB(funct3, rvcReg3(rs1), offset)
-				return []byte{byte(c16), byte(c16 >> 8)}, nil
-			}
-		}
-
 		word = riscvBType(enc, rs1, rs2, offset)
 
 	// U-type: rd, imm.
@@ -969,24 +949,19 @@ func tryCompressRVC(instr *ast.Instr, fi riscvFrameInfo) (uint16, bool) {
 		}
 
 	case "JAL":
-		// JAL X0, target → C.J when offset fits in ±2KB.
+		// JAL/JMP are never compressed to C.J by the Go assembler.
-		if len(ops) >= 1 {
-			// For JAL with implicit rd=0 (JMP alias), check target.
-			// C.J: funct3=0x5
-			// Offset is computed at encode time — we can't check it here.
 		return 0, false
-		}
 
 	case "JMP":
-		// C.J — handled in encodeRISCVInstr with actual offset.
+		// JAL/JMP are never compressed to C.J by the Go assembler.
 		return 0, false
 
 	case "BEQ":
-		// C.BEQZ — handled in encodeRISCVInstr with actual offset.
+		// Branches are never compressed to C.BEQZ/C.BNEZ.
 		return 0, false
 
 	case "BNE":
-		// C.BNEZ — handled in encodeRISCVInstr with actual offset.
+		// Branches are never compressed to C.BEQZ/C.BNEZ.
 		return 0, false
 
 	case "ADD":
@@ -1082,11 +1057,12 @@ func tryCompressRVC(instr *ast.Instr, fi riscvFrameInfo) (uint16, bool) {
 		}
 
 	case "LUI":
-		// LUI rd, imm → C.LUI when rd≠0, rd≠SP, imm nonzero and fits in 6 bits.
+		// LUI rd, imm → C.LUI when rd≠0, rd≠SP, imm nonzero and fits in six
+		// signed bits (matching the toolchain's compress pass).
 		if len(ops) == 2 {
 			rd := regFromOperand(ops[0])
 			imm := immFromOperand(ops[1])
-			if rd != -1 && rd != 0 && rd != 2 && imm != 0 && imm >= 1 && imm <= 63 {
+			if rd != -1 && rd != 0 && rd != 2 && imm != 0 && imm >= -32 && imm <= 31 {
 				return rvcCI(0x3, uint32(rd), uint32(imm)&0x3F), true
 			}
 		}

asm/riscv_encode.go

@@ -537,41 +537,12 @@ func rvcCIW(funct3, rd uint32, imm uint32) uint16 {
 	return uint16((funct3 << 13) | (packed << 5) | (rd << 2))
 }
 
-// rvcCJ encodes a CJ-type (jump) compressed instruction.
-// offset is a 12-bit signed offset (bit 0 is always 0).
-func rvcCJ(funct3 uint32, offset int32) uint16 {
-	uoff := uint32(offset) & 0xFFE
-	bits := ((uoff >> 11) & 1) << 10
-	bits |= ((uoff >> 4) & 1) << 9
-	bits |= ((uoff >> 9) & 0x3) << 7
-	bits |= ((uoff >> 10) & 1) << 6
-	bits |= ((uoff >> 6) & 1) << 5
-	bits |= ((uoff >> 7) & 1) << 4
-	bits |= ((uoff >> 1) & 0x7) << 1
-	bits |= ((uoff >> 5) & 1)
-	return uint16((funct3 << 13) | (bits << 2) | 0x1)
-}
-
 // rvcCA encodes a CA-type (arithmetic) compressed instruction.
 // Format: funct6[15:10] | rd'/rs1'[9:7] | funct2[6:5] | rs2'[4:2] | op=01.
 func rvcCA(funct6, funct2, rd, rs2 uint32) uint16 {
 	return uint16((funct6 << 10) | (rd << 7) | (funct2 << 5) | (rs2 << 2) | 0x1)
 }
 
-// rvcCB encodes a CB-type (branch) compressed instruction.
-// Format: funct3[15:13] | offset[8|4:3] | rs1'[9:7] | offset[7:6|2:1|5] | op=01.
-// Bit pattern for offset: [8|4:3|7:6|2:1|5]
-func rvcCB(funct3, rs1 uint32, offset int32) uint16 {
-	uoff := uint32(offset) & 0x1FE // bits [8:1]
-	offBits := uint32(0)
-	offBits |= ((uoff >> 8) & 1) << 10  // bit 10 = offset[8]
-	offBits |= ((uoff >> 3) & 0x3) << 8 // bits 9:8 = offset[4:3]
-	offBits |= ((uoff >> 6) & 0x3) << 6 // bits 7:6 = offset[7:6]
-	offBits |= ((uoff >> 1) & 0x3) << 3 // bits 4:3 = offset[2:1]
-	offBits |= ((uoff >> 5) & 1) << 2   // bit 2 = offset[5]
-	return uint16((funct3 << 13) | offBits | (rs1 << 7) | 0x1)
-}
-
 // rvcCBShift encodes a CB-type shift/immediate compressed instruction
 // (C.SRLI, C.SRAI, C.ANDI).  rd is the 3-bit prime-register index; imm is
 // the 6-bit shamt/immediate; funct2 selects the operation (0=SRLI, 1=SRAI,

asm/riscv_encode_test.go

@@ -428,7 +428,7 @@ TEXT ·`+tt.name+`(SB), NOSPLIT, $0
 }
 
 func TestRISCV_RVC_branch(t *testing.T) {
-	// BEQ rs, X0, target → C.BEQZ when rs is in prime regs and offset fits.
+	// Branches are never RVC-compressed (no C.BEQZ/C.BNEZ), matching go tool asm.
 	fn := firstTextRISCV(t, `#include "textflag.h"
 TEXT ·cbeqz(SB), NOSPLIT, $0
 	ADDI $1, X10, X10
@@ -438,14 +438,14 @@ done:
 	RET
 `)
 	code := assembleRISCVHelper(t, fn)
-	// C.ADDI(2) + C.BEQZ(2) + C.ADDI(2) + JALR(4) = 10 (all compress)
+	// C.ADDI(2) + BEQ(4) + C.ADDI(2) + JALR(4) = 12
-	if len(code) != 10 {
+	if len(code) != 12 {
-		t.Errorf("expected 10 bytes with C.BEQZ, got %d", len(code))
+		t.Errorf("expected 12 bytes with uncompressed BEQ, got %d", len(code))
 	}
 }
 
 func TestRISCV_RVC_CJ(t *testing.T) {
-	// JMP target → C.J when offset fits.
+	// JMP target → JAL X0 (never compressed to C.J), matching go tool asm.
 	fn := firstTextRISCV(t, `#include "textflag.h"
 	TEXT ·cj(SB), NOSPLIT, $0
 		JMP done
@@ -453,9 +453,9 @@ func TestRISCV_RVC_CJ(t *testing.T) {
 		RET
 	`)
 	code := assembleRISCVHelper(t, fn)
-	// C.J(2) + JALR(4) = 6
+	// JAL(4) + JALR(4) = 8
-	if len(code) != 6 {
+	if len(code) != 8 {
-		t.Errorf("expected 6 bytes with C.J, got %d", len(code))
+		t.Errorf("expected 8 bytes with uncompressed JMP, got %d", len(code))
 	}
 }
 

testdata/verify/branch_riscv64.s

@@ -0,0 +1,35 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+#include "textflag.h"
+
+TEXT ·branches(SB), NOSPLIT, $0
+	ADDI $1, X10, X10
+	BEQ X10, X11, beq_done
+	ADDI $2, X10, X10
+beq_done:
+	BNE X10, X11, bne_done
+	ADDI $3, X10, X10
+bne_done:
+	BLT X10, X11, blt_done
+	ADDI $4, X10, X10
+blt_done:
+	BGE X10, X11, bge_done
+	ADDI $5, X10, X10
+bge_done:
+	BLTU X10, X11, bltu_done
+	ADDI $6, X10, X10
+bltu_done:
+	BGEU X10, X11, bgeu_done
+	ADDI $7, X10, X10
+bgeu_done:
+	RET
+
+TEXT ·jumps(SB), NOSPLIT, $0
+	JMP done
+	ADDI $1, X10, X10
+done:
+	JAL X11, skip
+	ADDI $2, X10, X10
+skip:
+	RET

verify/riscv_groundtruth_test.go

@@ -23,6 +23,7 @@ func TestGroundTruthRISCV(t *testing.T) {
 		"../testdata/verify/loadstore_riscv64.s",
 		"../testdata/verify/largeimm_riscv64.s",
 		"../testdata/verify/movimm_riscv64.s",
+		"../testdata/verify/branch_riscv64.s",
 	} {
 		t.Run(path, func(t *testing.T) {
 			testGroundTruthRISCVFile(t, path)