[PATCH][ARM][RFC] PR target/65578 Fix gcc.dg/torture/stackalign/builtin-apply-4.c for single-precision fpus

Kyrill Tkachov kyrylo.tkachov@foss.arm.com
Tue Feb 9 17:24:00 GMT 2016


On 09/02/16 17:21, Kyrill Tkachov wrote:
> Hi all,
>
> In this wrong-code PR the builtin-apply-4.c test fails with -flto but only when targeting an fpu
> with only single-precision capabilities.
>
> bar is a function returing a double. For non-LTO compilation the caller of bar reads the return value
> from it from the s0 and s1 VFP registers like expected, but for -flto the caller seems to expect the
> return value from the r0 and r1 regs.  The RTL dumps show that too.
>
> Debugging the calls to arm_function_value show that in the -flto compilation the function bar is deemed
> to be a local function call and assigned the ARM_PCS_AAPCS_LOCAL PCS variant, whereas for the non-LTO (and non-breaking)
> compilation it uses the ARM_PCS_AAPCS_VFP variant.
>
> Further down in use_vfp_abi when deciding whether to use VFP registers for the result there is a bit of
> logic that rejects VFP registers when handling the ARM_PCS_AAPCS_LOCAL variant with a double precision value
> on an FPU that is not TARGET_VFP_DOUBLE.
>
> This seems wrong for ARM_PCS_AAPCS_LOCAL to me. ARM_PCS_AAPCS_LOCAL means that the function doesn't escape
> the translation unit and we can thus use whatever variant we want. From what I understand we want to use the
> VFP regs when possible for FP values.
>
> So this patch removes that restriction and for the testcase the caller of bar correctly reads the return
> value of bar from the VFP registers and everything works.
>
> This patch has been bootstrapped and tested on arm-none-linux-gnueabihf configured with --with-fpu=fpv4-sp-d16.
> The bootstrapped was performed with LTO.
> I didn't see any regressions.
>
> It seems that this logic was put there in 2009 with r154034 as part of a large patch to enable support for half-precision
> floating point.
>
> I'm not very familiar with this part of the code, so is this a safe patch to do?
> The patch should only ever change behaviour for single-precision-only fpus and only for static functions
> that don't get called outside their translation units (or during LTO I suppose) so there shouldn't
> be any ABI problems, I think.
>
> Is this ok for trunk?
>
> Thanks,
> Kyrill
>

Huh, I just realised I wrote completely the wrong PR number on this.
The PR I'm talking about here is PR target/69538

Sorry for the confusion.

Kyrill


> 2016-02-09 Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     PR target/65578
>     * config/arm/arm.c (use_vfp_abi): Remove id_double argument.
>     Don't check for is_double and TARGET_VFP_DOUBLE.
>     (aapcs_vfp_is_call_or_return_candidate): Update callsite.
>     (aapcs_vfp_is_return_candidate): Likewise.
>     (aapcs_vfp_is_call_candidate): Likewise.
>     (aapcs_vfp_allocate_return_reg): Likewise.



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