fix(asm): carry the riscv64 U-type immediate raw

The toolchain writes the source immediate straight into imm[31:12]
(riscv64.s: AUIPC 24287, X10 encodes 7ffff517), and rejects values
beyond the signed 20-bit span; the encoder divided by 4096 instead and
truncated silently, so the high bits of every large AUIPC and LUI were
lost.

Assisted-by: GLM 5.3 Flash

asm/riscv_assemble.go

@@ -1751,7 +1751,10 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		// The Go assembler never compresses branches to C.BEQZ/C.BNEZ.
 		word = riscvBType(enc, rs1, rs2, offset)
 
-	// U-type: rd, imm (or the toolchain testdata's INSTR $imm, rd).
+	// U-type: rd, imm (or the toolchain testdata's INSTR $imm, rd).  The
+	// immediate rides the field raw (riscv64.s: AUIPC $524287, X10 encodes
+	// 7ffff517), so it shifts into imm[31:12] here, and the span is the
+	// signed 20-bit range the toolchain checks.
 	case len(ops) == 2 && isUTypeInstr(mnem):
 		var rd int
 		var imm int32
@@ -1763,7 +1766,10 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		if rd < 0 {
 			return nil, fmt.Errorf("invalid register in %s", mnem)
 		}
-		word = riscvUType(enc, rd, imm)
+		if imm < -(1<<19) || imm > (1<<19)-1 {
+			return nil, fmt.Errorf("%s: signed immediate 0x%x must be in range [-0x80000, 0x7ffff] (20 bits)", mnem, imm)
+		}
+		word = riscvUType(enc, rd, imm<<12)
 
 	default:
 		return nil, fmt.Errorf("cannot encode %s with %d operands", mnem, len(ops))

asm/riscv_encode_test.go

@@ -109,6 +109,34 @@ TEXT ·imm(SB), NOSPLIT, $0
 	}
 }
 
+// TestRISCV_utypeImmediate pins the U-type immediate semantics byte for byte
+// against go tool asm (riscv64.s lines 76 to 85): the source immediate is the
+// raw 20-bit field, not a byte address to divide, and both sign extremes
+// encode.  The range beyond the signed 20 bits is the toolchain's own error.
+func TestRISCV_utypeImmediate(t *testing.T) {
+	fn := firstTextRISCV(t, `#include "textflag.h"
+TEXT ·utype(SB), NOSPLIT, $0
+	AUIPC $524287, X10
+	LUI $524287, X15
+	LUI $167, X15
+	AUIPC $-524288, X15
+	RET
+`)
+	code := assembleRISCVHelper(t, fn)
+	want := "17f5ff7f" + "b7f7ff7f" + "b7770a00" + "97070080"
+	if got := hex.EncodeToString(code[:16]); got != want {
+		t.Errorf("U-type immediates: %s, want %s", got, want)
+	}
+	fn = firstTextRISCV(t, `#include "textflag.h"
+TEXT ·wide(SB), NOSPLIT, $0
+	AUIPC $524288, X10
+	RET
+`)
+	if _, _, _, _, _, _, err := assembleRISCV(fn); err == nil {
+		t.Error("AUIPC $524288: expected the 20-bit range error, got none")
+	}
+}
+
 func TestRISCV_branches(t *testing.T) {
 	fn := firstTextRISCV(t, `#include "textflag.h"
 TEXT ·br(SB), NOSPLIT, $0