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

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

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

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.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index e71c9f56cbe846fdddf2e42a9f4575bacee570c1..e1404c74f74d01eb9c3362c7250e2b30ba5e47e7 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -5696,7 +5696,7 @@ aapcs_vfp_sub_candidate (const_tree type, machine_mode *modep)
 
 /* Return true if PCS_VARIANT should use VFP registers.  */
 static bool
-use_vfp_abi (enum arm_pcs pcs_variant, bool is_double)
+use_vfp_abi (enum arm_pcs pcs_variant)
 {
   if (pcs_variant == ARM_PCS_AAPCS_VFP)
     {
@@ -5715,8 +5715,7 @@ use_vfp_abi (enum arm_pcs pcs_variant, bool is_double)
   if (pcs_variant != ARM_PCS_AAPCS_LOCAL)
     return false;
 
-  return (TARGET_32BIT && TARGET_VFP && TARGET_HARD_FLOAT &&
-	  (TARGET_VFP_DOUBLE || !is_double));
+  return (TARGET_32BIT && TARGET_VFP && TARGET_HARD_FLOAT);
 }
 
 /* Return true if an argument whose type is TYPE, or mode is MODE, is
@@ -5758,7 +5757,7 @@ aapcs_vfp_is_call_or_return_candidate (enum arm_pcs pcs_variant,
     return false;
 
 
-  if (!use_vfp_abi (pcs_variant, ARM_NUM_REGS (new_mode) > 1))
+  if (!use_vfp_abi (pcs_variant))
     return false;
 
   *base_mode = new_mode;
@@ -5772,7 +5771,7 @@ aapcs_vfp_is_return_candidate (enum arm_pcs pcs_variant,
   int count ATTRIBUTE_UNUSED;
   machine_mode ag_mode ATTRIBUTE_UNUSED;
 
-  if (!use_vfp_abi (pcs_variant, false))
+  if (!use_vfp_abi (pcs_variant))
     return false;
   return aapcs_vfp_is_call_or_return_candidate (pcs_variant, mode, type,
 						&ag_mode, &count);
@@ -5782,7 +5781,7 @@ static bool
 aapcs_vfp_is_call_candidate (CUMULATIVE_ARGS *pcum, machine_mode mode,
 			     const_tree type)
 {
-  if (!use_vfp_abi (pcum->pcs_variant, false))
+  if (!use_vfp_abi (pcum->pcs_variant))
     return false;
 
   return aapcs_vfp_is_call_or_return_candidate (pcum->pcs_variant, mode, type,
@@ -5848,7 +5847,7 @@ aapcs_vfp_allocate_return_reg (enum arm_pcs pcs_variant ATTRIBUTE_UNUSED,
 			       machine_mode mode,
 			       const_tree type ATTRIBUTE_UNUSED)
 {
-  if (!use_vfp_abi (pcs_variant, false))
+  if (!use_vfp_abi (pcs_variant))
     return NULL;
 
   if (mode == BLKmode