[llvm-bugs] [Bug 37460] New: Missed rotate right optimization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37460

            Bug ID: 37460
           Summary: Missed rotate right optimization
           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,

LLVM doesn't optimize rotate right operation to ror x86 instruction for the
following case:

typedef struct { uint64_t state;  uint64_t inc; } pcg32_random_t;

uint32_t pcg32_random_r(pcg32_random_t* rng) {
    uint64_t oldstate = rng->state;
    rng->state = oldstate * 6364136223846793005ULL + (rng->inc|1);
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
}

Clang generates:
pcg32_random_r: # @pcg32_random_r
  mov rcx, qword ptr [rdi]
  mov rax, qword ptr [rdi + 8]
  movabs rdx, 6364136223846793005
  imul rdx, rcx
  or rax, 1
  add rax, rdx
  mov qword ptr [rdi], rax
  mov rax, rcx
  shr rax, 18
  xor rax, rcx
  shr rax, 27
  shr rcx, 59
  mov edx, eax
  shr edx, cl
  neg ecx
  shl eax, cl
  or eax, edx
  ret


Better code, generated from GCC:
pcg32_random_r:
  mov rcx, QWORD PTR [rdi]
  mov rdx, QWORD PTR [rdi+8]
  movabs rax, 6364136223846793005
  imul rax, rcx
  or rdx, 1
  add rax, rdx
  mov QWORD PTR [rdi], rax
  mov rax, rcx
  shr rax, 18
  xor rax, rcx
  shr rcx, 59
  shr rax, 27
  ror eax, cl
  ret



Source:
https://www.reddit.com/r/rust/comments/8jeybi/rust_does_not_rotate_or_rather_llvm/

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