[GHC] #11571: Need more intelligent conditionalization of libgcc rts symbols for x32

[GHC]

#11571: Need more intelligent conditionalization of libgcc rts symbols for x32
-------------------------------------+-------------------------------------
           Reporter:  rwbarton       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Runtime        |           Version:  8.1
  System (Linker)                    |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:  Other          |   Type of failure:  None/Unknown
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In `rts/RtsSymbols.c` we have
 {{{
 // 64-bit support functions in libgcc.a
 #if defined(__GNUC__) && SIZEOF_VOID_P <= 4 && !defined(_ABIN32)
 #define RTS_LIBGCC_SYMBOLS                             \
       SymI_NeedsProto(__divdi3)                        \
       SymI_NeedsProto(__udivdi3)                       \
       ...
 }}}
 The purpose is to provide these symbols to `.o` files that use these
 symbols, expecting to be linked against libgcc. These symbols are for
 things like 64-bit integer division that aren't available as machine
 instructions on some platforms.

 But the `#if` is wrong on x32, which, despite having `sizeof(void *) ==
 4`, naturally has a full complement of 64-bit arithmetic instructions,
 since its registers are 64 bits wide. So libgcc does not provide these
 symbols, and as a result we eventually encounter missing symbol errors
 when linking the rts.

 Some possible solutions include

 * Figure out if there is a CPP variable set for x32 and don't include
 these symbols if that variable is set. I'm not sure whether `__ILP32__` is
 such a variable (after all x86 is also an architecture on which int, long
 and pointers are 32-bits).

 * Use autoconf to detect the presence of these symbols in libgcc.
 (Probably just pick one, like `__divdi3`, and hope they are all present or
 all missing.)

 * Use weak symbols somehow to add these symbols to the RTS's internal
 symbol table if and only if the RTS was linked against a libgcc that
 provides these symbols.

 My Android ghci also needs lots more of these, including `__modsi3`,
 `__umodsi3`, `__ctzdi2`, a bunch of `__aeabi_*` and `__sync_*`, and `__sF`
 (no idea what that is). So I'm inclined towards the last option.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11571>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler