[ptr-compr] Use __builtin_assume_aligned() when available

... to let C++ compiler know that isolate root is 4Gb aligned and give
it a chance to generate a better code.

Bug: v8:9353
Change-Id: Ibd23c14cc44107c722a446a84dd14ca66f3bccfe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776079Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63445}

include/v8config.h

@@ -216,6 +216,7 @@
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (__has_attribute(warn_unused_result))
 
+# define V8_HAS_BUILTIN_ASSUME_ALIGNED (__has_builtin(__builtin_assume_aligned))
 # define V8_HAS_BUILTIN_BSWAP16 (__has_builtin(__builtin_bswap16))
 # define V8_HAS_BUILTIN_BSWAP32 (__has_builtin(__builtin_bswap32))
 # define V8_HAS_BUILTIN_BSWAP64 (__has_builtin(__builtin_bswap64))
@@ -262,6 +263,7 @@
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (!V8_CC_INTEL && V8_GNUC_PREREQ(4, 1, 0))
 
+# define V8_HAS_BUILTIN_ASSUME_ALIGNED (V8_GNUC_PREREQ(4, 7, 0))
 # define V8_HAS_BUILTIN_CLZ (V8_GNUC_PREREQ(3, 4, 0))
 # define V8_HAS_BUILTIN_CTZ (V8_GNUC_PREREQ(3, 4, 0))
 # define V8_HAS_BUILTIN_EXPECT (V8_GNUC_PREREQ(2, 96, 0))
@@ -300,6 +302,12 @@
 # define V8_INLINE inline
 #endif
 
+#if V8_HAS_BUILTIN_ASSUME_ALIGNED
+# define V8_ASSUME_ALIGNED(ptr, alignment) \
+  __builtin_assume_aligned((ptr), (alignment))
+#else
+# define V8_ASSUME_ALIGNED(ptr) (ptr)
+#endif
 
 // A macro used to tell the compiler to never inline a particular function.
 // Don't bother for debug builds.

src/common/ptr-compr-inl.h

@@ -35,7 +35,12 @@ V8_INLINE Address GetIsolateRoot<Address>(Address on_heap_addr) {
 
 template <>
 V8_INLINE Address GetIsolateRoot<Isolate*>(Isolate* isolate) {
-  return isolate->isolate_root();
+  Address isolate_root = isolate->isolate_root();
+#ifdef V8_COMPRESS_POINTERS
+  isolate_root = reinterpret_cast<Address>(V8_ASSUME_ALIGNED(
+      reinterpret_cast<void*>(isolate_root), kPtrComprIsolateRootAlignment));
+#endif
+  return isolate_root;
 }
 
 // Decompresses smi value.