[PATCH] D56325: Sort symbols in .bss by size.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu updated this revision to Diff 180287.
ruiu added a comment.

- fix comment


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D56325/new/

https://reviews.llvm.org/D56325

Files:
  lld/ELF/Writer.cpp
  lld/test/ELF/bss-sort.s
  lld/test/ELF/common.s


Index: lld/test/ELF/common.s
===================================================================
--- lld/test/ELF/common.s
+++ lld/test/ELF/common.s
@@ -18,7 +18,7 @@
 // CHECK-NEXT: AddressAlignment: 16
 
 // CHECK:      Name: sym1
-// CHECK-NEXT: Value: 0x201000
+// CHECK-NEXT: Value: 0x201014
 // CHECK-NEXT: Size: 8
 // CHECK-NEXT: Binding: Global
 // CHECK-NEXT: Type: Object
@@ -26,7 +26,7 @@
 // CHECK-NEXT: Section: .bss
 
 // CHECK:      Name: sym2
-// CHECK-NEXT: Value: 0x201008
+// CHECK-NEXT: Value: 0x20101C
 // CHECK-NEXT: Size: 8
 // CHECK-NEXT: Binding: Global
 // CHECK-NEXT: Type: Object
@@ -34,7 +34,7 @@
 // CHECK-NEXT: Section: .bss
 
 // CHECK:      Name: sym3
-// CHECK-NEXT: Value: 0x201010
+// CHECK-NEXT: Value: 0x201000
 // CHECK-NEXT: Size: 2
 // CHECK-NEXT: Binding: Global
 // CHECK-NEXT: Type: Object
@@ -42,7 +42,7 @@
 // CHECK-NEXT: Section: .bss
 
 // CHECK:      Name: sym4
-// CHECK-NEXT: Value: 0x201020
+// CHECK-NEXT: Value: 0x201010
 // CHECK-NEXT: Size: 4
 // CHECK-NEXT: Binding: Global
 // CHECK-NEXT: Type: Object
Index: lld/test/ELF/bss-sort.s
===================================================================
--- /dev/null
+++ lld/test/ELF/bss-sort.s
@@ -0,0 +1,19 @@
+// REQUIRES: x86
+// RUN: echo ".globl s4; .size arr, 4; .bss; s4: .zero 4" | \
+// RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t1.o
+// RUN: echo ".globl s8; .size arr, 8; .bss; s8: .zero 8" | \
+// RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t2.o
+// RUN: echo ".globl s1; .size arr, 1; .bss; s1: .zero 1" | \
+// RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t3.o
+
+// RUN: ld.lld -o %t.exe %t1.o %t2.o %t3.o
+// RUN: llvm-readelf -s %t.exe | FileCheck %s
+
+// Test that symbols in .bss are sorted by size.
+// CHECK: 0000000000201001     0 NOTYPE  GLOBAL DEFAULT    2 s4
+// CHECK: 0000000000201005     0 NOTYPE  GLOBAL DEFAULT    2 s8
+// CHECK: 0000000000201000     0 NOTYPE  GLOBAL DEFAULT    2 s1
+
+.global _start
+_start:
+  nop
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -1241,6 +1241,14 @@
   if (Name == ".init" || Name == ".fini")
     return;
 
+  // .bss contents are by default sorted by symbol size for better packing.
+  // According to Android Go project (a project to make Android work better
+  // on memory and CPU-constrained devices), it is found that this
+  // heuristics also achieves better memory write locality, which reduces
+  // number of dirty pages at runtime.
+  if (Name == ".bss")
+    Sec->sort([](InputSectionBase *S) { return S->getSize(); });
+
   // Sort input sections by priority using the list provided
   // by --symbol-ordering-file.
   if (!Order.empty())


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