PATCH combine.c bug: Bad to transform (cmpop (minus A B) 0) into (cmpop A B).

[Hans-Peter Nilsson]

> Date: Mon, 03 Apr 2000 22:57:03 -0600
> From: Jeffrey A Law <law@cygnus.com>

Thanks for reviewing this.

>   > 	* combine.c (simplify_comparison) [MINUS]: Do not replace
>   > 	all (op (minus A B) 0) with (op A B).
> This might be too pessimistic -- shouldn't we allow this transformation
> for any OP that doesn't depend on overflow?  That would be EQ, NE, GEU, GTU,
> LEU, and LTU?

As I mentioned, equality comparisons (EQ, NE) are handled by the
code after this deleted code.  Since you brought it up, I
thought I should check that the trivial transformations to
equality comparisons were performed for the unsigned
comparisons.  To test, I used the (now committed) test case,
tweaked the type of the operands and the comparison operator and
checked what landed in simplify_comparison.  It seems the right
conversions were done:

GEU is transformed to "always true"; simplify_comparison is
never called.

GTU is transformed to NE already at .00.rtl; first call to
simplify_comparison for these operands is with NE and the right
transformation is performed.

LEU is likewise transformed to EQ.
LTU is transformed to "always false".

Perhaps I should add this comment in place of the removed code
to avoid future confusion?

/* We used to optimize signed comparisons against zero, but that
   was incorrect.  Unsigned comparisons against zero (GEU, GTU,
   LEU, and LTU) arrive here as equality comparisons.  No need
   to special-case them.  */

Ok to commit with (or without) this comment?
brgds, H-P