Cr50: Add LLSR (long long shift right) support.

Cr50 lacks native instructions for 64-bit integers and an ABI
function can be used by the compiler to take the place of the
needed instructions. This CL adds support for a right bitwise
shift of 64-bit integers.

BRANCH=none
BUG=chromium:794010
TEST=Set CONFIG_LLSR_TEST, build, update cr50, and run llsrtest
on the console.

Change-Id: Iae66c86720c531454ba29f15b3cc6a07959f5ef2
Signed-off-by: Allen Webb <allenwebb@google.com>
Reviewed-on: https://chromium-review.googlesource.com/931932
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
(cherry picked from commit 6719bdf3ed)
Reviewed-on: https://chromium-review.googlesource.com/969632
Commit-Queue: Vadim Bendebury <vbendeb@chromium.org>
Tested-by: Vadim Bendebury <vbendeb@chromium.org>

core/cortex-m/build.mk

@@ -22,7 +22,7 @@ CFLAGS_CPU+=-flto
 LDFLAGS_EXTRA+=-flto
 endif
 
-core-y=cpu.o init.o ldivmod.o uldivmod.o
+core-y=cpu.o init.o ldivmod.o llsr.o uldivmod.o
 core-$(CONFIG_COMMON_PANIC_OUTPUT)+=panic.o
 core-$(CONFIG_COMMON_RUNTIME)+=switch.o task.o
 core-$(CONFIG_WATCHDOG)+=watchdog.o

core/cortex-m/llsr.c

@@ -0,0 +1,65 @@
+/* Copyright 2018 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/* Enable the use of right shift for uint64_t. */
+
+#include <console.h>
+#include <compile_time_macros.h>
+#include <stdint.h>
+
+union words {
+	uint64_t u64;
+	uint32_t w[2];
+};
+
+uint64_t __aeabi_llsr(uint64_t v, uint32_t shift)
+{
+	union words val;
+	union words res;
+
+	val.u64 = v;
+	res.w[1] = val.w[1] >> shift;
+	res.w[0] = val.w[0] >> shift;
+	res.w[0] |= val.w[1] >> (shift - 32); /* Handle shift >= 32*/
+	res.w[0] |= val.w[1] << (32 - shift); /* Handle shift <= 32*/
+	return res.u64;
+}
+
+#ifdef CONFIG_LLSR_TEST
+
+static int command_llsr(int argc, char **argv)
+{
+	/* Volatile to prevent compilier optimization from interfering. */
+	volatile uint64_t start = 0x123456789ABCDEF0ull;
+	uint32_t x;
+
+	const struct {
+		uint32_t shift_by;
+		uint64_t result;
+	} cases[] = {
+		{0, start},
+		{16, 0x123456789ABCull},
+		{32, 0x12345678u},
+		{48, 0x1234u},
+		{64, 0u}
+	};
+
+	for (x = 0; x < ARRAY_SIZE(cases); ++x) {
+		if ((start >> cases[x].shift_by) != cases[x].result) {
+			ccprintf("FAILED %d\n", cases[x].shift_by);
+			return EC_ERROR_UNKNOWN;
+		}
+	}
+
+	ccprintf("SUCCESS\n");
+	return EC_SUCCESS;
+}
+
+DECLARE_CONSOLE_COMMAND(
+		llsrtest, command_llsr,
+		"",
+		"Run tests against the LLSR ABI. Prints SUCCESS or FAILURE.");
+
+#endif  /* CONFIG_LLSR_TEST */

include/config.h

@@ -1749,6 +1749,12 @@
  */
 #undef CONFIG_LIGHTBAR_TAP_DIM_LAST_SEGMENT
 
+/*
+ * Adds a console command for testing the long long shift right ABI on Cortex-m4
+ * (Cr50).
+ */
+#undef CONFIG_LLSR_TEST
+
 /* Program memory offset for little firmware loader. */
 #undef CONFIG_LOADER_MEM_OFF