[llvm-bugs] [Bug 34517] New: Missed optimizations due to incomplete range analysis

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34517

            Bug ID: 34517
           Summary: Missed optimizations due to incomplete range analysis
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalobg88 at gmail.com
                CC: llvm-bugs at lists.llvm.org

Given this:

short fn1(short p) {
  short c = p / 32769;
  return c;
}

int fn2(signed char p) {
  int c = p / 129;
  return c;
}

int fn3(short p4, int p5) {
  int v = 0;
  if ((p4 - 32779) - !p5)
    v = 7;
  return v;
}

int fn4(int p, int q) {
  int a = q + (p % 6) / 9;
  return a;
}

int fn5(int p) {
  return (1 / p) / 100;
}

int fn6(int p5, int p6) {
  int a, c, v;
  v = -!p5;
  a = v + 7;
  c = (4 % a);
  return c;
}

int fn7(int p4, int p5) {
  int a = 5;
  if ((5 % p5) / 9)
    a = p4;
  return a;
}

GCC -O3 produces this output:

fn1(short):
  xor eax, eax
  ret
fn2(signed char):
  xor eax, eax
  ret
fn3(short, int):
  mov eax, 7
  ret
fn4(int, int):
  mov eax, esi
  ret
fn5(int):
  xor eax, eax
  ret
fn6(int, int):
  mov eax, 4
  ret
fn7(int, int):
  mov eax, 5
  ret

but clang -O3 produces this output:

fn1(short): # @fn1(short)
  movsxd rax, edi
  imul rax, rax, 1073709057
  mov rcx, rax
  shr rcx, 63
  shr rax, 45
  add eax, ecx
  ret
fn2(signed char): # @fn2(signed char)
  movsxd rax, edi
  imul rax, rax, 266354561
  mov rcx, rax
  shr rcx, 63
  sar rax, 35
  add eax, ecx
  ret
fn3(short, int): # @fn3(short, int)
  add edi, -32779
  xor eax, eax
  test esi, esi
  sete al
  xor ecx, ecx
  cmp edi, eax
  mov eax, 7
  cmove eax, ecx
  ret
fn4(int, int): # @fn4(int, int)
  movsxd rax, edi
  imul rcx, rax, 715827883
  mov rdx, rcx
  shr rdx, 63
  shr rcx, 32
  add ecx, edx
  add ecx, ecx
  lea ecx, [rcx + 2*rcx]
  sub eax, ecx
  cdqe
  imul rax, rax, 954437177
  mov rcx, rax
  shr rcx, 63
  sar rax, 33
  add eax, ecx
  add eax, esi
  ret
fn5(int): # @fn5(int)
  lea ecx, [rdi + 1]
  xor eax, eax
  cmp ecx, 2
  ja .LBB4_2
  movsxd rax, edi
  imul rax, rax, 1374389535
  mov rcx, rax
  shr rcx, 63
  sar rax, 37
  add eax, ecx
.LBB4_2:
  ret
fn6(int, int): # @fn6(int, int)
  cmp edi, 1
  mov ecx, 7
  sbb ecx, 0
  mov eax, 4
  xor edx, edx
  div ecx
  mov eax, edx
  ret
fn7(int, int): # @fn7(int, int)
  mov ecx, 5
  mov eax, 5
  xor edx, edx
  idiv esi
  cmp edx, 8
  cmova ecx, edi
  mov eax, ecx
  ret

See it live: https://godbolt.org/g/JwkE2Z

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