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From: "klaus.doldinger64 at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109476] New: Missing optimization for 8bit/8bit
 multiplication / regression
Date: Tue, 11 Apr 2023 19:19:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109476

            Bug ID: 109476
           Summary: Missing optimization for 8bit/8bit multiplication /
                    regression
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: klaus.doldinger64 at googlemail dot com
  Target Milestone: ---

For avr-gcc > 4.6.4 the follwing function

uint16_t mul(const uint8_t a, const uint16_t b) {
    return static_cast<uint8_t>((b >> 8) + 0) * a ;
}

produces suboptimal

mul(unsigned char, unsigned int):
        mov r18,r23
        ldi r19,0
        mov r20,r24
        mul r20,r18
        movw r24,r0
        mul r20,r19
        add r25,r0
        clr __zero_reg__
        ret

whereas avr-gcc 4.6.4 produces optimal

mul(unsigned char, unsigned int):
        mul r23,r24
        movw r24,r0
        clr r1
        ret=