[compiler-rt][builtins] Simplify __ucmpdi2

[Kayjukh]

Summary

When plugged into the current version of clang at optimization level O3,
the initial code produces an assembly output that looks like the
following on an x86 target:

__ucmpdi2:
  mov     rcx, rdi
  shr     rcx, 32
  mov     rdx, rsi
  shr     rdx, 32
  xor     eax, eax
  cmp     ecx, edx
  jb      .LBB0_4
  mov     eax, 2
  ja      .LBB0_4
  mov     eax, 0
  cmp     edi, esi
  jb      .LBB0_4
  cmp     esi, edi
  mov     eax, 1
  adc     eax, 0
.LBB0_4:
  ret

In constrast, the new version produces

__ucmpdi2:
  xor     eax, eax
  cmp     rdi, rsi
  seta    al
  sbb     eax, 0
  add     eax, 1
  ret

We can safely use arithmetic on booleans here, since both the C++ and C
standard guarantee that a bool to int conversion will produce either
zero (for false) or one (for true).

[efriedma]

LLVM never generates calls to __ucmpdi2, as far as I can tell. Do you have some other use-case?

[Kayjukh]

I don't have any specific use case. I think it may still be worth simplifying the function if it ever happens to be emitted by something.

compiler-rt/lib/builtins/ucmpdi2.c

 
 #include "int_lib.h"
 
 // Returns:  if (a <  b) returns 0
 //           if (a == b) returns 1
 //           if (a >  b) returns 2
 
 COMPILER_RT_ABI si_int __ucmpdi2(du_int a, du_int b) {
-  udwords x;
-  x.all = a;
-  udwords y;
-  y.all = b;
-  if (x.s.high < y.s.high)
-    return 0;
-  if (x.s.high > y.s.high)
-    return 2;
-  if (x.s.low < y.s.low)
-    return 0;
-  if (x.s.low > y.s.low)
-    return 2;
-  return 1;
+  return (a > b) - (a < b) + 1;
 }
 
 #ifdef __ARM_EABI__
-// Returns: if (a <  b) returns -1
+// Returns:  if (a <  b) returns -1
 //           if (a == b) returns  0
 //           if (a >  b) returns  1
 COMPILER_RT_ABI si_int __aeabi_ulcmp(di_int a, di_int b) {
   return __ucmpdi2(a, b) - 1;
 }
 #endif