[Bug tree-optimization/15826] don't use "if" to extract a single bit bit-field.

[matthew.fortune at imgtec dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15826

Matthew Fortune <matthew.fortune at imgtec dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |matthew.fortune at imgtec dot com

--- Comment #17 from Matthew Fortune <matthew.fortune at imgtec dot com> ---
(In reply to Bill Schmidt from comment #15)
> ;; Function andrew (andrew, funcdef_no=2, decl_uid=2361, cgraph_uid=2,
> symbol_order=2)
> 
> andrew (struct s * p)
> {
>   unsigned int _4;
>   unsigned int _5;
>   unsigned int _6;
> 
>   <bb 2>:
>   _4 = BIT_FIELD_REF <*p_3(D), 32, 0>;
>   _5 = _4 & 1;
>   _6 = _5;
>   return _6;
> 
> }

MIPS sees this same spurious failure. The issue (on a GCC 6 branch) is that the
optimization has successfully happened it just looks like the check is wrong in
the testcase. The presence of a logical and is not harmful as it is a
legitimate way to truncate: (from match.pd)

    /* A truncation to an unsigned type (a zero-extension) should be
       canonicalized as bitwise and of a mask.  */

This simplification kicks in for MIPS and presumably powerpc but does not (I
don't know why) for x86_64. There is no 'goto' demonstrating that a single bit
extract does not use an IF.

I therefore propose we just update the check to verify there is no 'goto'.

Matthew