[PATCH][rs6000] tighten checks before calling get_pool_constant

[Steven Bosscher]

Hi,

Without libcall notes we have failures on rs6000 that occur because
rs6000 accepts addresses for constant pool entries that the middle end
does not accept.

The failure occurs in rs6000_legitimize_reload_address which in the
patched compiler (no libcall notes) is at line 4192 and looks like:

 if (TARGET_TOC
    && constant_pool_expr_p (x)
    && ASM_OUTPUT_SPECIAL_POOL_ENTRY_P (get_pool_constant (x), mode))
  {
    x = create_TOC_reference (x);
    *win = 1;
    return x;
  }

The problem is that that rs6000.c:constant_pool_expr_p() is much more
general than varasm.c:get_constant_pool(). Consider:

(gdb) p debug_rtx (addr)
(const:DI (plus:DI (symbol_ref/u:DI ("*.LC169") [flags 0x2])
      (const_int 8 [0x8])))
$1 = void

constant_pool_expr_p() returns true this PLUS expressions, but
get_pool_constant only works on SYMBOL_REFs.  GCC ICEs with an RTL
accessor checking failure in SYMBOL_REF_CONSTANT.

The attached patch adds checks in rs6000.c in all places where
get_pool_constant() is called after constant_pool_expr_p().

Bootstrapped and tested on powerpc64-linux (-m32/-m64).  OK?

Gr.
Steven
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