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[avr] Document how to avoid progmem on AVR_TINY.

Message ID d490eee6-edc2-5b0d-e4a3-77a4721fa4d7@gjlay.de
State New
Headers show

Commit Message

Georg-Johann Lay Dec. 1, 2016, 2:28 p.m. UTC 
This adds to the documentation a hint how to set up a linker description 
file that avoids progmem altogether any without the usual overhead of 
locating read-only data in RAM.  The proposed linker description file is 
completely transparent to the compiler, and no start-up code has to be 
adjusted.

IIUC there are currently no plans to fix this in the default linker 
description file avrtiny.x, cf. http://sourceware.org/PR20849

Also, link between -mabsdata option and absdata variable attribute.

Ok for trunk?


Johann


gcc/
	* doc/invoke.texi (AVR Options) [-mabsdata]: Point to absdata.
	* doc/extend.texi (AVR Variable Attributes) [progmem]: Hint
	about linker description to avoid progmem altogether.
	[absdata]: Point to -mabsdata option.

Comments

Denis Chertykov Dec. 1, 2016, 4:48 p.m. UTC | #1 
2016-12-01 17:28 GMT+03:00 Georg-Johann Lay <avr@gjlay.de>:
> This adds to the documentation a hint how to set up a linker description
> file that avoids progmem altogether any without the usual overhead of
> locating read-only data in RAM.  The proposed linker description file is
> completely transparent to the compiler, and no start-up code has to be
> adjusted.
>
> IIUC there are currently no plans to fix this in the default linker
> description file avrtiny.x, cf. http://sourceware.org/PR20849
>
> Also, link between -mabsdata option and absdata variable attribute.
>
> Ok for trunk?
>
>
> Johann
>
>
> gcc/
>         * doc/invoke.texi (AVR Options) [-mabsdata]: Point to absdata.
>         * doc/extend.texi (AVR Variable Attributes) [progmem]: Hint
>         about linker description to avoid progmem altogether.
>         [absdata]: Point to -mabsdata option.
>

Approved.
diff mbox

Patch

Index: doc/extend.texi
===================================================================
--- doc/extend.texi	(revision 243111)
+++ doc/extend.texi	(working copy)
@@ -5929,6 +5929,30 @@  int read_var (int i)
 @}
 @end smallexample
 
+Please notice that on these devices, there is no need for @code{progmem}
+at all.  Just use an appropriate linker description file like outlined below.
+
+@smallexample
+  .text :
+  @{ ...
+  @} > text
+  /* Leave .rodata in flash and add an offset of 0x4000 to all
+     addresses so that respective objects can be accessed by LD
+     instructions and open coded C/C++.  This means there is no
+     need for progmem in the source and no overhead by read-only
+     data in RAM.  */
+  .rodata ADDR(.text) + SIZEOF (.text) + 0x4000 :
+  @{
+    *(.rodata)
+    *(.rodata*)
+    *(.gnu.linkonce.r*)
+  @} AT> text
+  /* No more need to put .rodata into .data:
+     Removed all .rodata entries from .data.  */
+  .data :
+  @{ ...
+@end smallexample
+
 @end table
 
 @item io
@@ -6001,6 +6025,8 @@  warning like
 
 @end itemize
 
+See also the @option{-mabsdata} @ref{AVR Options,command-line option}.
+
 @end table
 
 @node Blackfin Variable Attributes
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 243111)
+++ doc/invoke.texi	(working copy)
@@ -15402,7 +15402,8 @@  GCC supports the following AVR devices a
 
 Assume that all data in static storage can be accessed by LDS / STS
 instructions.  This option has only an effect on reduced Tiny devices like
-ATtiny40.
+ATtiny40.  See also the @code{absdata}
+@ref{AVR Variable Attributes,variable attribute}.
 
 @item -maccumulate-args
 @opindex maccumulate-args