[RFC AArch64] Implement TARGET_PROMOTE_FUNCTION_MODE for ILP32 code generation

[Yangfei (Felix)]

Hi Andrew, 

    Sorry for the late reply.  Seems that I misunderstood the AAPCS64 specification.  
    Thanks for the clarification.  


> 
> > On Mar 16, 2015, at 2:28 AM, Yangfei (Felix) <felix.yang@huawei.com> wrote:
> >
> > Hi,
> >
> >  For this trivial testcase:
> >
> > extern int bar (int , int);
> >
> > int foo (int *a, int *b)
> > {
> >    return bar (*a, *b);
> > }
> >
> > I noticed that GCC generate redundant zero-extension instructions under ILP32
> (aarch64-linux-gnu-gcc -S -O2 -mabi=ilp32).
> > Assembly code:
> >        .arch armv8-a+fp+simd
> >        .file   "1.c"
> >        .text
> >        .align  2
> >        .p2align 3,,7
> >        .global foo
> >        .type   foo, %function
> > foo:
> >        uxtw    x0, w0
> >        uxtw    x1, w1
> >        ldr     w0, [x0]
> >        ldr     w1, [x1]
> >        b       bar
> >        .size   foo, .-foo
> >        .ident  "GCC: (20140403) 5.0.0 20150312 (experimental)"
> >
> > According the ILP32 ABI, the two "uxtw" instrunctions here is not necessary.
> > The following is a proposed patch to fix this issue, i.e. promoting pointer
> function arguments to word_mode.
> > But I don't know whether it's a good idea to do this for pointer return values.
> > Any comments?
> 
> 
> Actually they are required. The abi says the upper 32bits are undefined for
> arguments smaller then 64bits. I had this discussion a year or more ago about this
> case.
> 
> A simple struct like
> struct a { int * b; int c; };
> 
> Will break the code if we don't have the zero extends
> 
> Try
> void f(int *);
> void g(struct a d)
> {
>   f(d.b);
> }
> 
> And see that there is no zero extends inside g.  I saw this exact thing when
> working on getting gccgo working.
> 
> It also means the assembly functions in glibc are broken and need to be fixed.
> 
> Thanks,
> Andrew
> >
> >
> > Index: gcc/config/aarch64/aarch64.c
> >
> =============================================================
> ======
> > --- gcc/config/aarch64/aarch64.c    (revision 221393)
> > +++ gcc/config/aarch64/aarch64.c    (working copy)
> > @@ -1587,7 +1587,7 @@ aarch64_function_value (const_tree type, const_tre
> >   machine_mode ag_mode;
> >
> >   mode = TYPE_MODE (type);
> > -  if (INTEGRAL_TYPE_P (type))
> > +  if (INTEGRAL_TYPE_P (type) || POINTER_TYPE_P (type))
> >     mode = promote_function_mode (type, mode, &unsignedp, func, 1);
> >
> >   if (aarch64_return_in_msb (type))
> > @@ -1650,6 +1650,24 @@ aarch64_function_value_regno_p (const unsigned
> int
> >   return false;
> > }
> >
> > +/* Implement TARGET_PROMOTE_FUNCTION_MODE.  */
> > +
> > +static machine_mode
> > +aarch64_promote_function_mode (const_tree type, machine_mode mode,
> > +                   int *punsignedp, const_tree fntype,
> > +                   int for_return)
> > +{
> > +  /* Pointer function arguments and return values are promoted to
> > +word_mode.  */
> > +  if (type != NULL_TREE && POINTER_TYPE_P (type))
> > +    {
> > +      *punsignedp = POINTERS_EXTEND_UNSIGNED;
> > +      return word_mode;
> > +    }
> > +
> > +  return default_promote_function_mode (type, mode, punsignedp, fntype,
> > +                                        for_return); }
> > +
> > /* Implement TARGET_RETURN_IN_MEMORY.
> >
> >    If the type T of the result of a function is such that @@ -11329,6
> > +11347,9 @@ aarch64_gen_adjusted_ldpstp (rtx *operands, bool l #define
> > TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE \
> >   aarch64_override_options_after_change
> >
> > +#undef TARGET_PROMOTE_FUNCTION_MODE
> > +#define TARGET_PROMOTE_FUNCTION_MODE
> aarch64_promote_function_mode
> > +
> > #undef TARGET_PASS_BY_REFERENCE
> > #define TARGET_PASS_BY_REFERENCE aarch64_pass_by_reference
> > <aarch64-promote-v2.diff>