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

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petrbalvin
2026-08-13 17:57:10 +02:00
parent 31a2cee382
commit 681a449c01
6 changed files with 62 additions and 72 deletions
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asm/riscv_assemble.go
+11 -35
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@@ -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.
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
}
}
// The Go assembler never compresses branches to C.BEQZ/C.BNEZ.
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.
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
}
// JAL/JMP are never compressed to C.J by the Go assembler.
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
}
}