fix(asm): compress movq immediates to the go-tool-asm imm32 forms

Assisted-by: GLM 5.3

asm/instrs.go

@@ -91,7 +91,28 @@ func (e *enc) encodeMov(ops []Operand, size int) error {
 
 	case Imm:
 		if dstIsReg {
-			// MOV r, imm: 0xB0+reg (8-bit) / 0xB8+reg (16/32/64, imm64 for Q).
+			v := int64(src)
+			// The Go assembler compresses 64-bit moves whose immediate fits
+			// a signed int32, choosing per sign:
+			//   v >= 0: B8+rd imm32 without REX.W (zero-extended by the
+			//           hardware, REX.B still emitted for R8-R15);
+			//   v <  0: REX.W C7 /0 imm32 (sign-extended — the plain B8+rd
+			//           form would zero-extend and corrupt the value).
+			// Out-of-range immediates keep the B8+rd imm64 form.
+			if size == 8 && v >= 0 && v <= (1<<31)-1 {
+				i := newInstr(4, []byte{0xB8 + byte(dstReg.idx&7)})
+				i.rexB = dstReg.idx >= 8
+				i.imm = le32(v)
+				return e.emit(i)
+			}
+			if size == 8 && v < 0 && v >= -(1<<31) {
+				i := newInstr(8, []byte{0xC7})
+				if err := setRMDigit(i, 0, dstReg, 8); err != nil {
+					return err
+				}
+				i.imm = le32(v)
+				return e.emit(i)
+			}
 			opBase := byte(0xB8)
 			if size == 1 {
 				opBase = 0xB0
@@ -101,7 +122,7 @@ func (e *enc) encodeMov(ops []Operand, size int) error {
 			if dstReg.needsREX(size) {
 				i.rexForced = true
 			}
-			i.imm = immediate(int64(src), size, true)
+			i.imm = immediate(v, size, true)
 			return e.emit(i)
 		}
 		// MOV r/m, imm: 0xC6 (8-bit) / 0xC7 /0.