From: Sergey Sharybin <sergey@blender.org>
Date: Wed, 9 Feb 2022 23:15:50 +0100
Subject: fix_Cycles_compilation_on_ARM

The rbit instruction is only available starting with ARMv6T2 and
the register prefix is different from what AARCH64 uses.

Separate the 32 and 64 bit ARM branches, add missing ISA checks.

Made sure the code works as intended on macMini with Apple silicon,
and on Raspberry Pi 4 B running 32bit Raspbian OS.

Differential Revision: https://developer.blender.org/D14056

Signed-off-by: Matteo F. Vescovi <mfv@debian.org>
---
 intern/cycles/util/math.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/intern/cycles/util/math.h b/intern/cycles/util/math.h
index 4e0e97f..7fc829f 100644
--- a/intern/cycles/util/math.h
+++ b/intern/cycles/util/math.h
@@ -849,9 +849,15 @@ ccl_device_inline uint prev_power_of_two(uint x)
 ccl_device_inline uint32_t reverse_integer_bits(uint32_t x)
 {
   /* Use a native instruction if it exists. */
-#if defined(__arm__) || defined(__aarch64__)
+#if defined(__aarch64__) || defined(_M_ARM64)
+  /* Assume the rbit is always available on 64bit ARM architecture. */
   __asm__("rbit %w0, %w1" : "=r"(x) : "r"(x));
   return x;
+#elif defined(__arm__) && ((__ARM_ARCH > 7) || __ARM_ARCH == 6 && __ARM_ARCH_ISA_THUMB >= 2)
+  /* This ARM instruction is available in ARMv6T2 and above.
+   * This 32-bit Thumb instruction is available in ARMv6T2 and above. */
+  __asm__("rbit %0, %1" : "=r"(x) : "r"(x));
+  return x;
 #elif defined(__KERNEL_CUDA__)
   return __brev(x);
 #elif __has_builtin(__builtin_bitreverse32)
