[llvm-bugs] [Bug 37368] New: Missed small switch optimizations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37368

            Bug ID: 37368
           Summary: Missed small switch optimizations
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

Hello,


When we have small "switch"-es with two or three cases, clang could generate a
smarter code.

Example:
char * a(int e) {
    switch (e) {
        case 0:
            return "0";
        default:
            return "default";
    }
}


Clang's output:
a: # @a
  push rbp
  mov rbp, rsp
  mov dword ptr [rbp - 12], edi
  mov edi, dword ptr [rbp - 12]
  test edi, edi
  jne .LBB0_2
  jmp .LBB0_1
.LBB0_1:
  movabs rax, offset .L.str
  mov qword ptr [rbp - 8], rax
  jmp .LBB0_3
.LBB0_2:
  movabs rax, offset .L.str.1
  mov qword ptr [rbp - 8], rax
.LBB0_3:
  mov rax, qword ptr [rbp - 8]
  pop rbp
  ret


Better code (GCC does it):
a:
  test edi, edi
  mov edx, OFFSET FLAT:.LC1
  mov eax, OFFSET FLAT:.LC0
  cmovne rax, rdx
  ret


Also for this example:
char * a(int e) {
    switch (e) {
        case 0:
            return "0";
        case 1:
            return "1";
        default:
            return "default";
    }
}

A better code can be generated like:
a:
  mov eax, OFFSET FLAT:.LC1
  test edi, edi
  je .L1
  cmp edi, 1
  mov eax, OFFSET FLAT:.LC2
  mov edx, OFFSET FLAT:.LC0
  cmovne rax, rdx
.L1:
  ret


Thanks

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