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arm assembly - generating of 32 bit constants

Chmielewski Andreas
Note: This was originally posted on 13th October 2009 at http://forums.arm.com

Hi,

i am searching for an algorithm to load any kind of 32 bit constants. As you know the arm compiler puts in the most cases the constants with some add/mul etc instructions together instead of loading the constant via the program counter out of the "constant pool". The way the compiler generates the constants is not really known to me. Unfortunately I could not find anything about it right now. Does somebody know where i can find something explaining whats the best way is to generate constants. Thx
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  • Arm Employee Badge
    Note: This was originally posted on 13th October 2009 at http://forums.arm.com

    For a generic 32-bit constants you are probably looking at out-of-line loads from a pc-relative literal pool - you cannot directly embed all 32-bit constant values in to an ARM instruction. You can embed *some* constants directly in to many instruction encodings - but these can only encode a limited set of values (8-bits rotated an event number of paces for most data processing instructions in the ARM instruction set).

    Thumb-2 introduces wide-data loads, allowing a pair of instructions to load the high and the low 16-bits of a register.

    For details of all of the instruction set encodings you probably want to get hold of the ARM Architecture Reference Manual - you can sign up for access using the link below:

    [url="https://silver.arm.com/browse/AR570"]https://silver.arm.com/browse/AR570[/url]
Reply
  • Arm Employee Badge
    Note: This was originally posted on 13th October 2009 at http://forums.arm.com

    For a generic 32-bit constants you are probably looking at out-of-line loads from a pc-relative literal pool - you cannot directly embed all 32-bit constant values in to an ARM instruction. You can embed *some* constants directly in to many instruction encodings - but these can only encode a limited set of values (8-bits rotated an event number of paces for most data processing instructions in the ARM instruction set).

    Thumb-2 introduces wide-data loads, allowing a pair of instructions to load the high and the low 16-bits of a register.

    For details of all of the instruction set encodings you probably want to get hold of the ARM Architecture Reference Manual - you can sign up for access using the link below:

    [url="https://silver.arm.com/browse/AR570"]https://silver.arm.com/browse/AR570[/url]
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