DJ Delorie <[EMAIL PROTECTED]> writes:
> I compared the generated code with an equivalent explicit test,
> and discovered that gcc uses a separate rtx for the intermediate:
>
> i = 0xfffff;
> if (j >= 16)
> {
> int i2;
> i2 = i >> 8;
> i = i2 >> 8;
> j -= 16;
> }
>
> This seems to avoid the combiner problem, becuase you don't have the
> same register being set and being used in one insn. Does this explain
> why combine was having a problem, or was this a legitimate thing to do
> and the combiner is still wrong? Using a temp in the expander works
> around the problem.
Interesting. Using a temporary is the natural way to implement this
code. But not using a temporary should be valid. So I think there is
a bug in combine.
But since using a temporary will give more CSE opportunities, I think
you should use a temporary. And you shouldn't worry about fixing
combine, since all that code is going to have to change on
dataflow-branch anyhow. (Actually it probably just works on
dataflow-branch.)
Ian
