[LTO] Make sure symbol ordering is honoured

[davide]

Summary

We do this emitting a section for every function when LTO is used.
See the discussion in https://bugs.llvm.org/show_bug.cgi?id=33888

I'll follow up with a similar change to the gold plugin. Other formats can probably do the same.

[ruiu]

Does this make sense to make true default?

[davide]

In D35809#819175, @ruiu wrote:

Does this make sense to make true default?

You mean, for the non-LTO case? Maybe, but I'd rather do it as a follow-up (as it needs more discussions/consensus).

[davide]

In D35809#819185, @davide wrote:

In D35809#819175, @ruiu wrote:

Does this make sense to make true default?

You mean, for the non-LTO case? Maybe, but I'd rather do it as a follow-up (as it needs more discussions/consensus).

Sorry, let me rephrase, fat-fingering. You mean, in the LTO API? Could be, but we need agreement across all the formats using this API (i.e. also COFF and Mach-O)

[ruiu]

No I mean we might want to set Conf.Options.FunctionSections to true by the ctor of lto::Config, so that that flag is true by default.

[ruiu]

Ah, makes sense. I think this looks good to me but please wait for other people who actually owns this one.

[pcc]

LGTM but I'd do the same for data-sections as well, so that we respect --gc-sections and --symbol-ordering-file for global variables.

Regarding other object formats, function-sections and data-sections have no effect on MachO because of their wacky subsections-via-symbols feature [1], so we only need to think about COFF. And I think this makes sense for COFF for the same reasons that it does for ELF. Feel free to deal with that in a followup, though.

[1] Or if we do the thing that James suggested in http://lists.llvm.org/pipermail/llvm-dev/2017-March/110763.html we would want to preserve the status quo by setting function-sections and data-sections to true when targeting MachO.

ELF/LTO.cpp

 
 static std::unique_ptr<lto::LTO> createLTO() {
   lto::Config Conf;
 
   // LLD supports the new relocations.
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
   Conf.Options.RelaxELFRelocations = true;
 
+  // Always emit a section per function with LTO.
+  Conf.Options.FunctionSections = true;
+
   if (Config->Relocatable)
     Conf.RelocModel = None;
   else if (Config->Pic)
     Conf.RelocModel = Reloc::PIC_;
   else
     Conf.RelocModel = Reloc::Static;
   Conf.CodeModel = GetCodeModelFromCMModel();
   Conf.DisableVerify = Config->DisableVerify;

test/ELF/lto/Inputs/symbol-ordering-lto.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-scei-ps4"
+
+define i32 @tin() {
+  ret i32 22
+}
+
+define i32 @pat() {
+  ret i32 42
+}

test/ELF/lto/symbol-ordering-lto.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-scei-ps4 %s -o %t.o
+# RUN: llvm-as %p/Inputs/symbol-ordering-lto.ll -o %t.bc
+
+# Set up the symbol file
+# RUN: echo "tin  " > %t_order_lto.txt
+# RUN: echo "_start " >> %t_order_lto.txt
+# RUN: echo "pat " >> %t_order_lto.txt
+
+# RUN: ld.lld --symbol-ordering-file %t_order_lto.txt %t.o %t.bc -o %t2.out
+# RUN: llvm-readobj -elf-output-style=GNU -t %t2.out| FileCheck %s
+
+# Check that the order is pat -> _start -> tin.
+
+# CHECK: Symbol table '.symtab' contains 4 entries:
+# CHECK-NEXT:   Num:    Value          Size Type    Bind   Vis      Ndx Name
+# CHECK-NEXT:     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
+# CHECK-NEXT:     1: 0000000000201008     0 NOTYPE  GLOBAL DEFAULT    1 _start
+# CHECK-NEXT:     2: 0000000000201020     6 FUNC    GLOBAL DEFAULT    1 pat
+# CHECK-NEXT:     3: 0000000000201000     6 FUNC    GLOBAL DEFAULT    1 tin
+
+.globl _start
+_start:
+  call pat
+  call tin