Re: PowerPC, small data operands and section names

[Thierry Moreau <Thierry dot Moreau at Connotech dot com>]

Thierry Moreau wrote:

> Dear GCC experts:
>
> Currently, in the gcc-3.x.x file gcc/config/rs6000/rs6000.c, with
> -msdata=sysv
> or -msdata=eabi, the compiler allocates the small objects of static
> storage
> duration (global variables) to loader sections .sbss, .sdata, .sbss2,
> .sdata2.
> The size threshold is specified with the -G compiler option.
>
> In addition to section allocation, the compiler uses efficient load and
> store
> instruction sequences (with the @sda21 or @sdarel operand relocation
> modifiers)
> for these small objects. The maximum displacement is 32K from symbols
> _SDA_BASE... This is part of the PowerPC (E)ABI specifications.
>
> Here is an improvement suggestion: I would like the compiler to base its
>
> decision about efficient instructions on the actual object section, not
> on the
> size of small objects.
>
> When small objects are explicitly allocated to named sections (e.g. in a
>
> peripheral memory section), the linker may complain that the @sda21 or
> @sdarel
> relocation modifiers are not within the 32KB limit. The suggestion would
> fix
> this.
>
> In addition, this would give the programmer greater control over which
> objects
> are accessed efficiently.
>
> I identified the function small_data_operand in the file
> gcc/config/rs6000/rs6000.c as the decision to use the @sda21 or @sdarel
> relocation modifiers (other uses of the variable g_switch_value is where
> the
> section allocation decision is made).

Ooops, this does not seem to be the case: the function
rs6000_encode_section_info
determines if an object name is subject to the @sda21 or @sdarel relocation
modifiers,
and prefixes a '@' cheracter to the symbol name if yes (as an indication to
be used by the
function small_data_operand). However, the section name test in
rs6000_encode_section_info
does not seem to work. I'm investigating this.

> I don't understand how the RTL
> data (type
> rtx) manipulated when emitting assembler instructions is related to the
> "tree"
> type used when allocating data declarations to sections.
>
> Main Question: Is there a way to retrieve the "tree" of a symbol from
> the "rtx"
> of the load/store instruction? From this, It should then be possible to
> base the
> decision in small_data_operand on section information rather than on the
> size
> information.
>
> Basically, the PowerPC (E)ABI reserves the %r2 and/or  %r13 registers
> for
> "system" use. The above suggestion would give the embedded developer
> greater
> freedom and control over their use.
>
> Many thanks for your attention.

--

- Thierry Moreau

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