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From: "goon.pri.low at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112645] New: missed-optimization: cswitch
 optimization missed in nested if-statement
Date: Tue, 21 Nov 2023 01:21:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112645

            Bug ID: 112645
           Summary: missed-optimization: cswitch optimization missed in
                    nested if-statement
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goon.pri.low at gmail dot com
  Target Milestone: ---

These two functions should theoretically generate the same code, though the
second one uses a constant array.

int a(int v) {
    switch (v & 2) {
    case 0: //0x
        switch(v & 1) {
        case 0: //00
            return 643;
        case 1: //01
            return 223;
        }
    case 2: //1x
        switch (v & 1) {
        case 0: //10
            return 444;
        case 1: //11
            return 532;
        }
    }
}

a:
        test    dil, 2
        je      .L7
        and     edi, 1
        cmp     edi, 1
        sbb     eax, eax
        and     eax, -88
        add     eax, 532
        ret
.L7:
        and     edi, 1
        cmp     edi, 1
        sbb     eax, eax
        and     eax, 420
        add     eax, 223
        ret

int b(int v) {
    switch (v & 3) {
    case 0: //00
        return 643;
    case 1: //01
        return 223;
    case 2: //10
        return 444;
    case 3: //11
        return 532;
    }
}

b:
        and     edi, 3
        mov     eax, 643
        sub     edi, 1
        cmp     edi, 2
        ja      .L8
        mov     eax, DWORD PTR CSWTCH.2[0+rdi*4]
.L8:
        ret
CSWTCH.2:
        .long   223
        .long   444
        .long   532

Additionally while testing this, I found this function which should just us=
e a
simple binary and

int c(int v) {
    switch (v & 3) {
    case 0:
        return 0;
    case 1:
        return 1;
    case 2:
        return 2;
    case 3:
        return 3;
    }
}

c:
        mov     eax, edi
        and     eax, 3
        lea     edx, [rax-1]
        cmp     edx, 3
        mov     edx, 0
        cmovnb  eax, edx
        ret

though has an unnecessary comparison and conditional move?=