[PATCH] D31443: [LTO] Do not reorder global variables unnecessarily during merging

[Tobias Edler von Koch via Phabricator via llvm-commits]

tobiasvk updated this revision to Diff 93317.
tobiasvk added a comment.

Make Gerolf's suggested changes to comments.


https://reviews.llvm.org/D31443

Files:
  lib/Object/ModuleSymbolTable.cpp
  test/LTO/Resolution/X86/globalorder.ll


Index: test/LTO/Resolution/X86/globalorder.ll
===================================================================
--- /dev/null
+++ test/LTO/Resolution/X86/globalorder.ll
@@ -0,0 +1,28 @@
+; Check that LTO keeps global variables in source code order
+;
+; RUN: llvm-as %s -o %t.bc
+; RUN: llvm-lto2 %t.bc -O0 -save-temps -o %t.o -r %t.bc,var1,px -r %t.bc,var2,px -r %t.bc,var3,px -r %t.bc,var4,px -r %t.bc,foo,px
+; RUN: llvm-dis -o - %t.o.0.0.preopt.bc | FileCheck %s
+;
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: @var1 =
+ at var1 = global i32 0, align 4
+; CHECK: @var2 =
+ at var2 = global i32 0, align 4
+; CHECK: @var3 =
+ at var3 = global i32* @var1, align 4
+; CHECK: @var4 =
+ at var4 = global i32* @var2, align 4
+
+define i32 @foo() {
+entry:
+  %0 = load i32*, i32** @var3, align 4
+  %1 = load i32, i32* %0, align 4
+  %2 = load i32*, i32** @var4, align 4
+  %3 = load i32, i32* %2, align 4
+  %add = add nsw i32 %3, %1
+  ret i32 %add
+}
Index: lib/Object/ModuleSymbolTable.cpp
===================================================================
--- lib/Object/ModuleSymbolTable.cpp
+++ lib/Object/ModuleSymbolTable.cpp
@@ -43,10 +43,14 @@
   else
     FirstMod = M;
 
-  for (Function &F : *M)
-    SymTab.push_back(&F);
+  // The iteration order here is significant as it relates to the order in which
+  // globals are materialized during module merging in regular LTO.  Variables
+  // need to be added first so their original ordering is not changed
+  // unnecessarily.
   for (GlobalVariable &GV : M->globals())
     SymTab.push_back(&GV);
+  for (Function &F : *M)
+    SymTab.push_back(&F);
   for (GlobalAlias &GA : M->aliases())
     SymTab.push_back(&GA);
 


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