aarch64: Remove redundant zero-extends with LDAR

Like other loads in AArch64, the LDARB,LDARH,LDAR instructions clear out the top part of their
destination register and we can thus avoid having to explicitly zero-extend it.
We were missing a combine pattern that this patch adds.

For one of the examples in the testcase we generated:
load_uint8_t_ext_uint16_t:
        adrp    x0, .LANCHOR0
        add     x0, x0, :lo12:.LANCHOR0
        ldarb   w0, [x0]
        and     w0, w0, 255
        ret

but now generate:
load_uint8_t_ext_uint16_t:
        adrp    x0, .LANCHOR0
        add     x0, x0, :lo12:.LANCHOR0
        ldarb   w0, [x0]
        ret

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/atomics.md (*atomic_load<ALLX:mode>_zext<SD_HSDI:mode>):
	New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/ldar_2.c: New test.

1 files changed, 27 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.target/aarch64/ldar_2.c b/gcc/testsuite/gcc.target/aarch64/ldar_2.c
new file mode 100644
index 0000000..60b0717
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/ldar_2.c
@@ -0,0 +1,27 @@
+/* Test that the zero-extending patterns for LDAR are used.  */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+#include <stdint.h>
+
+uint8_t v_uint8_t;
+uint16_t v_uint16_t;
+uint32_t v_uint32_t;
+uint64_t v_uint64_t;
+
+#define FUNC(FROM, TO)						\
+TO								\
+load_##FROM##_ext_##TO (void)					\
+{								\
+  return __atomic_load_n (&v_##FROM, __ATOMIC_ACQUIRE);		\
+}
+
+FUNC (uint8_t, uint16_t)
+FUNC (uint8_t, uint32_t)
+FUNC (uint8_t, uint64_t)
+FUNC (uint16_t, uint32_t)
+FUNC (uint16_t, uint64_t)
+FUNC (uint32_t, uint64_t)
+
+/* { dg-final { scan-assembler-not {and\tw[0-9+], w[0-9]+, 255} } } */
+/* { dg-final { scan-assembler-not {uxtw\tx[0-9+], w[0-9]+} } } */