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diff --git a/powerpc64/README b/powerpc64/README new file mode 100644 index 00000000..19351be8 --- /dev/null +++ b/powerpc64/README @@ -0,0 +1,73 @@ +General-Purpose Register Conventions + +Register Status Use + +GPR0 volatile In function prologs. +GPR1 dedicated Stack pointer. +GPR2 dedicated Table of Contents (TOC) pointer. +GPR3 volatile First word of a function's argument list; + first word of a scalar function return. +GPR4 volatile Second word of a function's argument list; + second word of a scalar function return. +GPR5 volatile Third word of a function's argument list. +GPR6 volatile Fourth word of a function's argument list. +GPR7 volatile Fifth word of a function's argument list. +GPR8 volatile Sixth word of a function's argument list. +GPR9 volatile Seventh word of a function's argument list. +GPR10 volatile Eighth word of a function's argument list. +GPR11 volatile In calls by pointer and as an environment pointer + for languages that require it (for example, PASCAL). +GPR12 volatile For special exception handling required by certain + languages and in glink code. +GPR13 reserved Reserved under 64-bit environment; + not restored across system calls. +GPR14:GPR31 nonvolatile These registers must be preserved across + a function call. + +Vector Register Conventions + +Register Status + +VR0:V19 Volatile +VR20:VR31 Nonvolatile (extended ABI mode) their values are preserved + across function calls + +Addressing memory + +There are many ways to reference data, in the sake of writing +position-independent code the current implementation uses GOT-indirect +addressing (Accessing data through the global offset table): +1. Define data in .data section +2. Load the address of data into register from the global offset table + e.g. ld 7, my_var@got(2) +3. Use the address to load the value of data into register + e.g. ld 3, 0(7) +Refer to [2] for more information about referencing data + +VSX instructions "lxvd2x/stxvd2x" are used to load and store data to +memory instead of VR instructions "lvx/stvx" as it produces a fewer +instructions "lvx/stvx" can be used to load/store data into storage +operands but additional instructions are needed to access unaligned +storage operands, refer to "6.4.1 Accessing Unaligned Storage Operands" +in [3] to see an example of accessing unaligned storage operands. +"lxvd2x/stxvd2x" can be used to load/store data into unaligned storage +operands but permuting is needed for loading and storing data in +little-endian mode VSX registers are defined with "X" suffix +TODO: use architecture 3.0 instructions "lxv/stxv" instead for POWER9 + and newer + +Function Prologue + +Big-endian systems only support ELFv1 ABI which requires the following +steps in the function prologue: +1. Write the "official procedure descriptor" in ".opd","aw" section +2. Write procedure description for .my_func in my_func label +3. Switch back to ".text" section for program code +4. Label the beginning of the code .my_func +Refer to [1] for more information +Little-endian systems are compatible with ELFv2 ABI, an example of +function prologue for ELFv2 ABI can be seen in [2] + +[1] http://www.ibm.com/developerworks/linux/library/l-powasm1.html +[2] https://openpowerfoundation.org/?resource_lib=64-bit-elf-v2-abi-specificatio... +[3] https://openpowerfoundation.org/?resource_lib=ibm-power-isa-version-2-07-b