[PATCH][arm] PR target/82518: Return false in ARRAY_MODE_SUPPORTED_P for BYTES_BIG_ENDIAN

[Kyrill Tkachov]

Hi all,

This PR shows that we get the load/store_lanes logic wrong for arm big-endian.
It is tricky to get right. Aarch64 does it by adding the appropriate 
lane-swapping
operations during expansion.

I'd like to do the same on arm eventually, but we'd need to port and validate 
the VTBL-generating
code and add it to all the right places and I'm not comfortable enough doing it 
for GCC 8, but I am keen
in getting the wrong-code fixed.
As I say in the PR, vectorisation on armeb is already severely restricted (we 
disable many patterns on BYTES_BIG_ENDIAN)
and the load/store_lanes patterns really were not working properly at all, so 
disabling them is not
a radical approach.

The way to do that is to return false in ARRAY_MODE_SUPPORTED_P for 
BYTES_BIG_ENDIAN.

Bootstrapped and tested on arm-none-linux-gnueabihf.
Also tested on armeb-none-eabi.

Committing to trunk.
Thanks,
Kyrill

2018-03-20  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    PR target/82518
    * config/arm/arm.c (arm_array_mode_supported_p): Return false for
    BYTES_BIG_ENDIAN.

2018-03-20  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    PR target/82518
    * lib/target-supports.exp (check_effective_target_vect_load_lanes):
    Disable for armeb targets.
    * gcc.target/arm/pr82518.c: New test.

commit 861f9881d3af7e6f94e9b008e88aa7ba7c44a4df
Author: Kyrylo Tkachov <kyrylo.tkac...@arm.com>
Date:   Tue Feb 20 15:14:01 2018 +0000

    [arm] PR target/82518: Return false in ARRAY_MODE_SUPPORTED_P for BYTES_BIG_ENDIAN

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 73e9fc1..09deb60 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -27151,7 +27151,10 @@ static bool
 arm_array_mode_supported_p (machine_mode mode,
 			    unsigned HOST_WIDE_INT nelems)
 {
-  if (TARGET_NEON
+  /* We don't want to enable interleaved loads and stores for BYTES_BIG_ENDIAN
+     for now, as the lane-swapping logic needs to be extended in the expanders.
+     See PR target/82518.  */
+  if (TARGET_NEON && !BYTES_BIG_ENDIAN
       && (VALID_NEON_DREG_MODE (mode) || VALID_NEON_QREG_MODE (mode))
       && (nelems >= 2 && nelems <= 4))
     return true;
diff --git a/gcc/testsuite/gcc.target/arm/pr82518.c b/gcc/testsuite/gcc.target/arm/pr82518.c
new file mode 100644
index 0000000..c3e45b8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr82518.c
@@ -0,0 +1,29 @@
+/* { dg-do run } */
+/* { dg-require-effective-target arm_neon_ok } */
+/* { dg-additional-options "-O3 -fno-inline -std=gnu99" } */
+/* { dg-add-options arm_neon } */
+
+typedef struct { int x, y; } X;
+
+void f4(X *p, int n)
+{
+  for (int i = 0; i < n; i++)
+  { p[i].x = i;
+    p[i].y = i + 1;
+  }
+}
+
+__attribute ((aligned (16))) X arr[100];
+
+int main(void)
+{
+  volatile int fail = 0;
+  f4 (arr, 100);
+  for (int i = 0; i < 100; i++)
+    if (arr[i].y != i+1 || arr[i].x != i)
+      fail = 1;
+  if (fail)
+     __builtin_abort ();
+
+  return 0;
+}
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index 640f76e..5ff5ec6 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -6609,7 +6609,8 @@ proc check_effective_target_vect_load_lanes { } {
 	verbose "check_effective_target_vect_load_lanes: using cached result" 2
     } else {
 	set et_vect_load_lanes 0
-	if { ([istarget arm*-*-*] && [check_effective_target_arm_neon_ok])
+	# We don't support load_lanes correctly on big-endian arm.
+	if { ([istarget arm-*-*] && [check_effective_target_arm_neon_ok])
 	     || [istarget aarch64*-*-*] } {
 	    set et_vect_load_lanes 1
 	}