[Bug ld/25861] xtensa ld error: (.debug_line+0x3b12): dangerous relocation: overflow after relaxation

[jcmvbkbc at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=25861

--- Comment #3 from jcmvbkbc at gcc dot gnu.org ---
It occurs because our treatment of XTENSA_DIFF16 relocation is inconsistent.
Assembler is free to use this relocation with both positive and negative values
up to 16 bit wide, linker always treats is as signed 16-bit value, i.e. it only
has 15 bits of value.

In the failing build the assembler has .loc directive that updates current
address from 0 to 0x8bb7. It writes positive value 0x8bb7 there, linker treats
it as 0xffffffffffff8bb7, then it adds this value to the subtracted symbol
address (which is 0) and uses 0xffffffffffff8bb7 as an address of the
diminished symbol. There's no check that the address is within the section
limits (that would have stopped it right there), so it proceeds with relaxation
adjustment, and finds where this address moves after relaxation. Relaxation
removes more than 32K from the section, so relaxed address is
0xfffffffffffe714b, which doesn't fit into 16 bits, that's where build error is
reported. In reality the  address 0x8bb7 was turned to 0x1db, which is
perfectly fine.

So XTENSA_DIFF relocations don't supply enough information to deal with both
unsigned and signed 16-bit values. Proposed fix is to deprecate XTENSA_DIFF
signed relocations and not generate them any more in the assembler. Instead
introduce XTENSA_PDIFF relocations for positive differences (subtracted symbol
precedes diminished symbol) and XTENSA_NDIFF for negative differences
(subtracted symbol follows diminished symbol). The difference value itself is
treated as unsigned in both cases. The linker will continue to support
XTENSA_DIFF as it did before and in addition will support XTENSA_PDIFF and
XTENSA_NDIFF with new semantic.

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