[PATCH][GCC][ARM] Fix can_change_mode_class for big-endian

[Tamar Christina <tamar dot christina at arm dot com>]

Hi All,

This patch requires https://gcc.gnu.org/ml/gcc-patches/2018-06/msg01145.html to work,
it has been accepted once already but caused a regression on certain configuratoins.
I am re-submitting it with the required mid-end change and requesting a back-port.

--- original patch.

Taking the subreg of a vector mode on big-endian may result in an infinite
recursion and eventually a segfault once we run out of stack space.

As an example, taking a subreg of V4HF to SImode we end up in the following
loop on big-endian:

#861 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787
#862 0x0000000000882a90 in emit_move_multi_word src/gcc/gcc/expr.c:3621
#863 0x000000000087eea1 in emit_move_insn_1 src/gcc/gcc/expr.c:3698
#864 0x000000000087f350 in emit_move_insn src/gcc/gcc/expr.c:3757
#865 0x000000000085e326 in copy_to_reg src/gcc/gcc/explow.c:603
#866 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787

The reason is that operand_subword_force will always fail. When the value is in
a register that can't be accessed as a multi word the code tries to create a new
psuedo register and emit the value to it. Eventually you end up in simplify_gen_subreg
which calls validate_subreg.

validate_subreg will however always fail because of the REG_CAN_CHANGE_MODE_P check.

On little endian this check always returns true. On big-endian this check is supposed
to prevent values that have a size larger than word size, due to those being stored in
VFP registers.

However we are only interested in a subreg of the vector mode, so we should be checking
the unit size, not the size of the entire mode. Doing this fixes the problem.

Regtested on armeb-none-eabi and no regressions.
Bootstrapped on arm-none-linux-gnueabihf and no issues.

Ok for trunk? and for backport to GCC 8?

Thanks,
Tamar

gcc/
2018-06-19  Tamar Christina  <tamar.christina@arm.com>

	PR target/84711
	* config/arm/arm.c (arm_can_change_mode_class): Use GET_MODE_UNIT_SIZE
	instead of GET_MODE_SIZE when comparing Units.
	
gcc/testsuite/
2018-06-19  Tamar Christina  <tamar.christina@arm.com>

	PR target/84711
	* gcc.target/arm/big-endian-subreg.c: New.

-- 

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 90d62e699bce9594879be2e3016c9b36c7e064c8..703632240822e762a906570964b949c783df56f3 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -31508,8 +31508,8 @@ arm_can_change_mode_class (machine_mode from, machine_mode to,
 {
   if (TARGET_BIG_END
       && !(GET_MODE_SIZE (from) == 16 && GET_MODE_SIZE (to) == 8)
-      && (GET_MODE_SIZE (from) > UNITS_PER_WORD
-	  || GET_MODE_SIZE (to) > UNITS_PER_WORD)
+      && (GET_MODE_UNIT_SIZE (from) > UNITS_PER_WORD
+	  || GET_MODE_UNIT_SIZE (to) > UNITS_PER_WORD)
       && reg_classes_intersect_p (VFP_REGS, rclass))
     return false;
   return true;
diff --git a/gcc/testsuite/gcc.target/arm/big-endian-subreg.c b/gcc/testsuite/gcc.target/arm/big-endian-subreg.c
new file mode 100644
index 0000000000000000000000000000000000000000..4b1ab122f349e61e296fe3f76030a7a258e55f62
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/big-endian-subreg.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target arm_neon_ok } */
+/* { dg-require-effective-target arm_hf_eabi } */
+/* { dg-add-options arm_neon } */
+/* { dg-additional-options "-mfp16-format=ieee -mfloat-abi=hard" } */
+
+typedef __fp16 v4f16
+  __attribute__ ((vector_size (8)));
+
+v4f16 fn1 (v4f16 p)
+{
+  return p;
+}