CODEWITHSUNDEEP
assemblygccarmgnu-assemblerimmediate-operand
Benny
Benny

Reputation: 4321

arm asm: Value too large for field with numeric literal for :upper8_15:

GNU ARM assembler supports prefixes to extract group of bytes from a relocation value https://sourceware.org/binutils/docs/as.html#ARM_002dRelocations

On ARMv7m and later, the low/high 16 bit operators work fine, including with literal numbers not just symbol addresses.

    movw r0, #:lower16:0xabadcafe
    movt r0, #:upper16:0xabadcafe

But I cannot manage to get the 8 bits prefixes working with literals, on any arch (v6m, v7m, v8m):

       .thumb
       .text
test:
       movs    r0, #:upper8_15:0xabadcafe

Compiling this code returns an error; which is weird to me because the prefix is apparently ok with the parser (I was expecting a bad expression error), but instead is just doing nothing as the full 32 bits value is considered by the movs instruction.

$ ./arm-gnu-toolchain-12.2.rel1-x86_64-arm-none-eabi/bin/arm-none-eabi-gcc -o test -mcpu=cortex-m23 -march=armv8-m.base -mthumb  -mpure-code -c test.S 

test.S: Assembler messages:
test.S:5: Error: invalid immediate: -1414673666 is out of range
test.S:5: Error: value of abadcafe too large for field of 2 bytes at 00000000

I made some research online and apparently this is supposed to work; the snippet in https://www.mail-archive.com/[email protected]/msg230076.html is exactly what I want to achieve on v6m: loading a register without any data access using the prefix feature of the assembler.

Upvotes: 1

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