[codegen] Align the code start at 64 byte in x64

In order to make loop header aligned at 64 byte (relative to memory address), code start should also be aligned at 64 byte.

Bug: chromium:1231471
Change-Id: I95390babd9cc78492e0beb0f1b03901eb481d5d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3094167Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Hao A Xu <hao.a.xu@intel.com>
Cr-Commit-Position: refs/heads/main@{#76484}

src/common/globals.h

@@ -590,9 +590,14 @@ constexpr intptr_t kPointerAlignmentMask = kPointerAlignment - 1;
 constexpr intptr_t kDoubleAlignment = 8;
 constexpr intptr_t kDoubleAlignmentMask = kDoubleAlignment - 1;
 
-// Desired alignment for generated code is 32 bytes (to improve cache line
-// utilization).
+// Desired alignment for generated code is 64 bytes on x64 (to allow 64-bytes
+// loop header alignment) and 32 bytes (to improve cache line utilization) on
+// other architectures.
+#if V8_TARGET_ARCH_X64
+constexpr int kCodeAlignmentBits = 6;
+#else
 constexpr int kCodeAlignmentBits = 5;
+#endif
 constexpr intptr_t kCodeAlignment = 1 << kCodeAlignmentBits;
 constexpr intptr_t kCodeAlignmentMask = kCodeAlignment - 1;
 

src/objects/code.h

@@ -547,7 +547,7 @@ class Code : public HeapObject {
 #elif V8_TARGET_ARCH_LOONG64
   static constexpr int kHeaderPaddingSize = 24;
 #elif V8_TARGET_ARCH_X64
-  static constexpr int kHeaderPaddingSize = COMPRESS_POINTERS_BOOL ? 12 : 24;
+  static constexpr int kHeaderPaddingSize = COMPRESS_POINTERS_BOOL ? 12 : 56;
 #elif V8_TARGET_ARCH_ARM
   static constexpr int kHeaderPaddingSize = 12;
 #elif V8_TARGET_ARCH_IA32

src/snapshot/embedded/platform-embedded-file-writer-aix.cc

@@ -65,8 +65,14 @@ void PlatformEmbeddedFileWriterAIX::DeclareSymbolGlobal(const char* name) {
 }
 
 void PlatformEmbeddedFileWriterAIX::AlignToCodeAlignment() {
+#if V8_TARGET_ARCH_X64
+  // On x64 use 64-bytes code alignment to allow 64-bytes loop header alignment.
+  STATIC_ASSERT((1 << 6) >= kCodeAlignment);
+  fprintf(fp_, ".align 6\n");
+#else
   STATIC_ASSERT((1 << 5) >= kCodeAlignment);
   fprintf(fp_, ".align 5\n");
+#endif
 }
 
 void PlatformEmbeddedFileWriterAIX::AlignToDataAlignment() {

src/snapshot/embedded/platform-embedded-file-writer-generic.cc

@@ -74,8 +74,14 @@ void PlatformEmbeddedFileWriterGeneric::DeclareSymbolGlobal(const char* name) {
 }
 
 void PlatformEmbeddedFileWriterGeneric::AlignToCodeAlignment() {
+#if V8_TARGET_ARCH_X64
+  // On x64 use 64-bytes code alignment to allow 64-bytes loop header alignment.
+  STATIC_ASSERT(64 >= kCodeAlignment);
+  fprintf(fp_, ".balign 64\n");
+#else
   STATIC_ASSERT(32 >= kCodeAlignment);
   fprintf(fp_, ".balign 32\n");
+#endif
 }
 
 void PlatformEmbeddedFileWriterGeneric::AlignToDataAlignment() {

src/snapshot/embedded/platform-embedded-file-writer-mac.cc

@@ -56,12 +56,18 @@ void PlatformEmbeddedFileWriterMac::DeclareSymbolGlobal(const char* name) {
   // prevents something along the compilation chain from messing with the
   // embedded blob. Using .global here causes embedded blob hash verification
   // failures at runtime.
-  STATIC_ASSERT(32 >= kCodeAlignment);
   fprintf(fp_, ".private_extern _%s\n", name);
 }
 
 void PlatformEmbeddedFileWriterMac::AlignToCodeAlignment() {
+#if V8_TARGET_ARCH_X64
+  // On x64 use 64-bytes code alignment to allow 64-bytes loop header alignment.
+  STATIC_ASSERT(64 >= kCodeAlignment);
+  fprintf(fp_, ".balign 64\n");
+#else
+  STATIC_ASSERT(32 >= kCodeAlignment);
   fprintf(fp_, ".balign 32\n");
+#endif
 }
 
 void PlatformEmbeddedFileWriterMac::AlignToDataAlignment() {