[llvm-bugs] [Bug 30273] New: [InstCombine] failed to convert selects into add

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30273

            Bug ID: 30273
           Summary: [InstCombine] failed to convert selects into add
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

This example is a simplified test based on actual code in
InstCombineCompares.cpp as of r280624:

$ cat boolcow.c

#include <stdbool.h>

unsigned foo(bool a, bool b) {
  unsigned ret = 0;
  if (a) ++ret;
  if (b) ++ret;
  return ret;
}


This is logically equivalent in C99 or C++ to:

unsigned goo(bool a, bool b) {
  return a + b;
}

It seems that there's a missing transform in InstCombine itself because the IR
for these two cases is:

$ ./clang -O1 boolcow.c -S -o - -emit-llvm
define i32 @foo(i1 %a, i1 %b) {
  %. = zext i1 %a to i32
  %inc4 = select i1 %a, i32 2, i32 1
  %ret.1 = select i1 %b, i32 %inc4, i32 %.
  ret i32 %ret.1
}

define i32 @goo(i1 %a, i1 %b) {
  %conv = zext i1 %a to i32
  %conv3 = zext i1 %b to i32
  %add = add nuw nsw i32 %conv3, %conv
  ret i32 %add
}

...so fix this bug, and the compiler might get faster. :)

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