[llvm-commits] [pr12251][patch] Use i1* for boolean loads and stores

[Rafael Espíndola]

In the testcase of pr12251 we use a i8* for loading a boolean value:

  %3 = load i8* %2
  %4 = trunc i8 %3 to i

The problem then is that LLVM has no way of knowing that %3 can only
be 0 or 1. A C file using uint8_t would have compiled to the same IL.

It would be nice to have a general way of telling LLVM that a value is
known to be in a range, but for the specific cases of booleans we can
do better by just using a more restrictive type. This patch changes
clang to use i1 pointers for doing the loads and stores. This changes
the generated code from

    movb    (%rdi), %al
    andb    $1, %al
    ret

to just

    movb    (%rdi), %al
    ret

I think the idea can be extended for systems that use 32 bit booleans.
We would just have to be careful with endianess. I got the idea from
http://llvm.org/bugs/show_bug.cgi?id=8730#c3.

Is this patch OK?

Cheers,
Rafael
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