Re: CSE bug when narrowing constants

[Gary Funck <gary at intrepid dot com>]

On 11/29/08 14:45:49, Eric Botcazou wrote:
> > Agreed.  The routine that creates the errant REG_EQUAL note is
> > lookup_as_function().
> 
> Really?  Doesn't it only retrieve a pre-existing REG_EQUAL note?

It retrieves an equivalent rtx constant, if it exists.  Before the patch,
the constant that was returned is a word mode (DImode) constant
with the value 0x100000000 (33 bits), which won't fit into an SImode
value, and therefore isn't equivalent.  The fix is to call gen_lowpart()
in the case where the word mode constant is narrowed to a smaller mode.
In the example, the lower 32 bits of the constant will be used,
which is 0, and is the correct equivalent constant.  cse_insn() calls
lookup_as_function() ultimately through fold_rtx(), IIRC, and is the
routine that writes the REG_EQUAL note.