Re: CSE bug when narrowing constants

[Gary Funck <gary at intrepid dot com>]

On 12/01/08 11:50:48, Eric Botcazou wrote:
> > cse_insn() calls lookup_as_function() ultimately through fold_rtx(), IIRC,
> > and is the routine that writes the REG_EQUAL note.
> 
> OK, thanks.  But I'm a little at a loss as to why this problem arises only
> now: the problematic code in lookup_as_function is one decade old.  Do you 
> happen to have older compilers around (say GCC 4.1.x based) that correctly 
> compile the testcase?  If so, what happens differently with them?

Yeah, I was surprised as well.  The compiler base line
this problem arose on is 4.2.3, but I think that it will
occur in both older and newer base lines.  The problem is
triggered by code generated by the UPC (Unified Parllel C)
support that we've implemented in a project we call GCC/UPC.
It fails on a small UPC test case, but a number
of factors have to be present to trigger the problem.
I tried developing a vanilla C test case to duplicate the
problem, but have so far been unsuccessful.

Internally, we use VIEW_CONVERT_EXPR to overlay a TImode
container on top of a struct.  There is no exact C
equivalent, though a union comes close.  I tried that,
but couldn't replicate the exact set of events that have
to be present to hit the problem. I send what I tried
to you separately.  Perhaps adding some
sort of logging in lookup_as_function() that indicates
narrowing is occurring, and then running all test cases
(including Ada, because its unchecked_conversion is close
to what we're doing internally) would turn something up?