[LLVMdev] InstCombine adds bit masks, confuses self, others

[Chandler Carruth]

On Wed, Apr 18, 2012 at 12:22 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

> I tried disabling just the InstCombine transforms that hide shl
> instructions behind bitmasks. Even though DAGCombine has the same
> transforms, it causes some pretty bad regressions:
>

I wonder about your idea of still combining most of the shifts, but leaving
the last bits uncombined... Certainly the combining effects seem
particularly important to capture because we'll only look through a finite
number of shifts to determine demanded bits, etc.

I would expect that, even with the fix, demanded bits might need adjusting
to cope with the pattern.

But this is all theorizing. =] The actually regressions will likely tell
more.
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