> On Mar 16, 2015, at 2:28 AM, Yangfei (Felix) <felix.y...@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>
