RTL alias analysis

[Mark Mitchell]

Steven Bosscher wrote:
> Hi rth,
> 
> The stack space sharing you added to cfgexpand.c breaks RTL alias
> analysis.
> 
> For example, the attached test case breaks for pentiumpro at -O2. 
> The problem apparently is that the second store to c is moved up
> before before the load.

My guess at a solution is that when A (with alias set S_a) and B (with
alias set S_b) are given the same stack slot, we should create a new
alias set S_c which is a subset of both S_a and S_b, and give the
combined stack slot that aliase set.

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Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
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