Re: [PATCH] Disable aggregate jump functions for bit-field stores

[Jan Hubicka <hubicka at ucw dot cz>]

> Hi,
> 
> the patch below disables generation of aggregate jump functions from
> bit-field stores because currently we depend on type size of the value
> to determine the size of the stored value and that does not work with
> bit-fields, making it impossible for IPA-CP to organize them in their
> lattices.
> 
> If we ever decide aggregate jump functions for bit-fields are worth
> the hassle, we might remove this limitation by storing and streaming
> the size of the memory reference alongside the offset in the jump
> functions (and IPA-CP lattices).
> 
> Bootstrapped and tested on x86_64-linux, needed for the aggregate
> IPA-CP.  OK for trunk?
> 
> Thanks,
> 
> Martin
> 
> 
> 2012-11-02  Martin Jambor  <mjambor@suse.cz>
> 
> 	* ipa-prop.c (determine_known_aggregate_parts): Do not create
> 	aggregate jump functions for bit-fields.
> 
> Index: src/gcc/ipa-prop.c
> ===================================================================
> --- src.orig/gcc/ipa-prop.c
> +++ src/gcc/ipa-prop.c
> @@ -1295,7 +1295,10 @@ determine_known_aggregate_parts (gimple
>  
>        lhs = gimple_assign_lhs (stmt);
>        rhs = gimple_assign_rhs1 (stmt);
> -      if (!is_gimple_reg_type (rhs))
> +      if (!is_gimple_reg_type (rhs)
> +          || TREE_CODE (lhs) == BIT_FIELD_REF
> +	  || (TREE_CODE (lhs) == COMPONENT_REF
> +	      && DECL_BIT_FIELD (TREE_OPERAND (lhs, 1))))

I am not sure I understand motivation of this patch properly.  First I think
BIT_FIELD_REF can be hidden inside chain of other REFs so you should probably
look into all handled components.  What exactly goes wrong when your type size
is bigger than the bitfield?

Honza