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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hello All,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In ARM embedded applications, there are some compilers that support useful function and variable attributes that help the compiler communicate information about symbols to downstream object consumers (i.e. linkers).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One such attribute is the “location” attribute. This attribute can be applied to a global or local static data object or a function to indicate to the linker that the definition of the data object or function should be placed at a specific
address in memory.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For example, in the following code:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">#include <stdio.h><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">extern int a;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">int a __attribute__((location(0x1000))) = 4;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">struct bstruct<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">{<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> int f1;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> int f2;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">struct bstruct b __attribute__((location(0x1004))) = {10, 12};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">double c __attribute__((location(0x1010))) = 1.0;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">char d[] __attribute__((location(0x2000))) = {1, 2, 3, 4};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">void foo(double x) __attribute((location(0x4000)));<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">void foo(double x) { printf("%f\n", x); }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D">A location attribute has been applied to several data objects and the function “foo.” The compiler would then encode information into the compiled object file that tells the downstream linker about these memory
placement constraints on the data objects and function.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D">Without extending the ELF object format, how would this work?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D">I propose to encode metadata information about a symbol in special absolute symbols, “__sym_attr_metadata.<int>”, that the linker can recognize when scanning the symbol table for an incoming object file. In an
ELF symbol table entry:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">typedef struct {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf32_Word st_name;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf32_Addr st_value;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf32_Word st_size;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> unsigned char st_info;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> unsigned char st_other;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf32_Half st_shndx;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">} Elf32_Sym;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">typedef struct {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf64_Word st_name;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> unsigned char st_info;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> unsigned char st_other;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf64_Half st_shndx;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf64_Addr st_value;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> Elf64_Xword st_size;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D">} Elf64_Sym;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D">The st_size and st_value fields could be used to represent attribute information about a given symbol:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#172B4D"><o:p> </o:p></span></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="mso-list:l0 level1 lfo1">The st_size field can be split into an attribute ID and a symbol index for the symbol that the attribute applies to<o:p></o:p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="mso-list:l0 level2 lfo1">attribute ID: bits 0..7<o:p></o:p></li><li class="MsoNormal" style="mso-list:l0 level2 lfo1">symbol index: bits 8..31<o:p></o:p></li></ul>
</li><li class="MsoNormal" style="mso-list:l0 level1 lfo1">The st_value field can contain the value associated with the attribute (i.e. the address argument of a location attribute)<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If the compiler is generating assembly code, a new directive similar to the .eabi_attribute can be used:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Courier New";color:#172B4D"> .symbol_attribute <symbol name>, <attribute kind>, <attribute value><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Where:<o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="mso-list:l1 level1 lfo2"><b>symbol name</b> - will unambiguously identify the symbol that the attribute/value pair applies to<o:p></o:p></li><li class="MsoNormal" style="mso-list:l1 level1 lfo2"><b>attribute kind</b> - is an unsigned integer between 1 and 255 that specifies the kind of attribute to be applied to the symbol<o:p></o:p>
<ul style="margin-top:0in" type="disc">
<li class="MsoNormal" style="mso-list:l1 level2 lfo2">I propose a starting base set of 2 attribute IDs: used (1), location (2)
<o:p></o:p></li><li class="MsoNormal" style="mso-list:l1 level2 lfo2">the compiler will emit the integer constant that identifies the attribute kind<o:p></o:p></li></ul>
</li><li class="MsoNormal" style="mso-list:l1 level1 lfo2"><b>attribute value </b>- a value that is appropriate for the specified attribute kind<o:p></o:p></li></ul>
<p class="MsoNormal" style="margin-left:.25in"><o:p> </o:p></p>
<p class="MsoNormal">Thoughts? Comments? Concerns?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The anticipated next steps would be to add support for the location attribute and update the ARM/ELF LLVM back-end to support encoding the used attribute with the new mechanism.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">~ Todd Snider<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Code Generation Tools Group<o:p></o:p></p>
<p class="MsoNormal">Texas Instruments Incorporated<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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