Assembler: subtracting symbols in different sections

Question

Hello,
the x86(_64) assembler that comes with Sun Studio refuses to compute the difference between symbols in different sections with an error message: "Illegal subtraction - symbols from different sections".
Even if it makes sense to refuse such subtractions (is there a standard somewhere that says they are illegal? The ELF ABI maybe?), gnu tools accept them, so people start using them (apparently there can be some benefits), and the code fails to compile with Sun Studio. I know gcc regularly has such problems since around 2005, and the latest version of gmp also has this issue.
Is there a chance such subtractions may become legal in some not too distant future?

Answer 1

The GNU assembler manual says that a program cannot subtract
symbols from different sections. The text states
Subtraction. If the right argument is absolute, the result has the section of
the left argument. If both arguments are in the same section, the result is
absolute. You may not subtract arguments from different sections.
I wrote some trial programs that subtracted symbols from different sections.
The assembler accepted one of them, but most failed to assemble with error
messages such as
t.s: Assembler messages:
t.s:27: Error: can't resolve `a' {.data section} - `main' {.text section}
There is a reason to reject such expressions. There is no ELF relocation type
for such expressions.
The one program that succeeded in assembling produced the value zero for
the subtraction even though it should have been nonzero.
I ran my tests under SLES 10 SP2. I used the assembler in /usr/bin/as. It is
possible that other Linux distros would produce different results.
Bob Corbett