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diff --git a/arch/h8300/lib/mulsi3.S b/arch/h8300/lib/mulsi3.S
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+++ b/arch/h8300/lib/mulsi3.S
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+;
+; mulsi3 for H8/300H - based on Renesas SH implementation
+;
+; by Toshiyasu Morita
+;
+; Old code:
+;
+; 16b * 16b = 372 states (worst case)
+; 32b * 32b = 724 states (worst case)
+;
+; New code:
+;
+; 16b * 16b =  48 states
+; 16b * 32b =  72 states
+; 32b * 32b =  92 states
+;
+
+	.global __mulsi3
+__mulsi3:
+	mov.w	r1,r2   ; ( 2 states) b * d
+	mulxu	r0,er2  ; (22 states)
+
+	mov.w	e0,r3   ; ( 2 states) a * d
+	beq	L_skip1 ; ( 4 states)
+	mulxu	r1,er3  ; (22 states)
+	add.w	r3,e2   ; ( 2 states)
+
+L_skip1:
+	mov.w	e1,r3   ; ( 2 states) c * b
+	beq	L_skip2 ; ( 4 states)
+	mulxu	r0,er3  ; (22 states)
+	add.w	r3,e2   ; ( 2 states)
+
+L_skip2:
+	mov.l	er2,er0	; ( 2 states)
+	rts		; (10 states)
+
+	.end