On the Cortex M0, does the multiply set the flags. That way, if one is writing a 32x32=64 bit multiply, a fast exit can be added. I'm just looking at this code I found and wondered if it were worth inserting.
+ /* Slow version for both THUMB and older ARMs lacking umull. */
+ mul xxh, yyl /* xxh := AH*BL */
+ push {r4, r5, r6, r7}
+ mul yyh, xxl /* yyh := AL*BH */
+ ldr r4, .L_mask
+ lsr r5, xxl, #16 /* r5 := (AL>>16) */
+ lsr r6, yyl, #16 /* r6 := (BL>>16) */
+ lsr r7, xxl, #16 /* r7 := (AL>>16) */
+ mul r5, r6 /* r5 = (AL>>16) * (BL>>16) */
+ and xxl, r4 /* xxl = AL & 0xffff */
+ and yyl, r4 /* yyl = BL & 0xffff */
+ add xxh, yyh /* xxh = AH*BL+AL*BH */
+ mul r6, xxl /* r6 = (AL&0xffff) * (BL>>16) */
+ mul r7, yyl /* r7 = (AL>>16) * (BL&0xffff) */
+ add xxh, r5
+ mul xxl, yyl /* xxl = (AL&0xffff) * (BL&0xffff) */
+ mov r4, #0
+ adds r6, r7 /* partial sum to result[47:16]. */
+ adc r4, r4 /* carry to result[48]. */
+ lsr yyh, r6, #16
+ lsl r4, r4, #16
+ lsl yyl, r6, #16
+ add xxh, r4
+ adds xxl, yyl
+ adc xxh, yyh
+ pop {r4, r5, r6, r7}
+ RET
+ .align 2
+.L_mask:
+ .word 65535
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