[codegen] Use target architecture specific branch alignment

Stephan Herhut · Commit Bot · commit be213cfc4851 · 2019-01-23T11:50:25.000Z
The Assembler class (or some of them at least) have a CodeTargetAlign method that aligns the code to a target specific value (16 byte on x86, 8 byte on arm). However, these were not used. Instead we always aligned to 16 byte boundaries, hence wasting up to 8 bytes on arm. Change-Id: Iee7d24ebc13a9a58002a9d7d0ce53955bee7d628 Reviewed-on: https://chromium-review.googlesource.com/c/1426125 Commit-Queue: Stephan Herhut <herhut@chromium.org> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org> Cr-Commit-Position: refs/heads/master@{#59024}
diff --git a/src/arm64/assembler-arm64.cc b/src/arm64/assembler-arm64.cc
@@ -621,6 +621,10 @@ void Assembler::Align(int m) {
   }
 }
 
+void Assembler::CodeTargetAlign() {
+  // Preferred alignment of jump targets on some ARM chips.
+  Align(8);
+}
 
 void Assembler::CheckLabelLinkChain(Label const * label) {
 #ifdef DEBUG
diff --git a/src/arm64/assembler-arm64.h b/src/arm64/assembler-arm64.h
@@ -287,6 +287,8 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   // Insert the smallest number of zero bytes possible to align the pc offset
   // to a mulitple of m. m must be a power of 2 (>= 2).
   void DataAlign(int m);
+  // Aligns code to something that's optimal for a jump target for the platform.
+  void CodeTargetAlign();
 
   inline void Unreachable();
 
diff --git a/src/compiler/backend/code-generator.cc b/src/compiler/backend/code-generator.cc
@@ -191,9 +191,9 @@ void CodeGenerator::AssembleCode() {
 
   // Assemble instructions in assembly order.
   for (const InstructionBlock* block : code()->ao_blocks()) {
-    // Align loop headers on 16-byte boundaries.
+    // Align loop headers on vendor recommended boundaries.
     if (block->ShouldAlign() && !tasm()->jump_optimization_info()) {
-      tasm()->Align(16);
+      tasm()->CodeTargetAlign();
     }
     if (info->trace_turbo_json_enabled()) {
       block_starts_[block->rpo_number().ToInt()] = tasm()->pc_offset();

Original file line number	Diff line number	Diff line change
`@@ -621,6 +621,10 @@ void Assembler::Align(int m) {`
`621`	`621`	`}`
`622`	`622`	`}`
`623`	`623`
	`624`	`+void Assembler::CodeTargetAlign() {`
	`625`	`+ // Preferred alignment of jump targets on some ARM chips.`
	`626`	`+ Align(8);`
	`627`	`+}`
`624`	`628`
`625`	`629`	`void Assembler::CheckLabelLinkChain(Label const * label) {`
`626`	`630`	`#ifdef DEBUG`