Is it possible to delay resolving a .weakreference in gcc-as until link time? (GCC)

Question

If you want to declare .weakreference for a set of interrupt vectors such as:

.weakreference ISR_SPI_HANDLER, defaultExceptionHandler

in a startup.S file or some framework include, it's very useful except that if you then declare ISR_SPI_HANDLER as a function in your main.c file, ISR_SPI_HANDLER in the startup.S still resolves to defaultExceptionHandler because .weakreference only works in local scope (terminology? pre-linking). However, .weak functions as it's expected, and allows you to "override" the definition with a definition in main.c, but if you don't, you'll always resolve to 0x00, which is inferior obviously to resolving to a default vector.

How can I overcome this?

Post your alternate solution as a separate answer, not as part of the question, please. — Peter Cordes

artless noise artless noise · Accepted Answer · 2017-10-03T14:57:27

Do not use the .weakref in the assembler. Instead just use ISR_SPI_HANDLER as an external. The assembler naturally wants to optimize to PC relative jumps to known locations. You are fighting this and someone else will want the call to defaultExceptionHandler.

Instead use the linker,

  .text : {
    *(.text);
    ISR_SPI_HANDLER = DEFINED(ISR_SPI_HANDLER) ? ISR_SPI_HANDLER : defaultExceptionHandler;
    …
  }

Alternatively, split startup.S with the .weakref and defaultExceptionHandler declaration in one file and another that is using/calling ISR_SPI_HANDLER.

Is it possible to delay resolving a .weakreference in gcc-as until link time? (GCC)

2 Answers