Need more intelligent conditionalization of libgcc rts symbols for x32

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.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System (Linker)
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