Prefer -Ttext over linker script values

[• rafael]

Summary

I found this while trying to build u-boot. It uses -Ttext in combination with linker scripts.

My first reaction was to change the linker scripts to have the correct value, but I found that it is actually quite convenient to have -Ttext take precedence.

By having just

.text : { *(.text) }

In the script, they can define the text address in a single Makefile and pass it to ld with -Ttext and for the C code with -DFoo=value. Doing the same with linker scripts would require them to be generated during the build.

[emaste]

I like this idea.

[ruiu]

LGTM

[• espindola]

305766

[• espindola]

305766

ELF/LinkerScript.cpp

void LinkerScript::fabricateDefaultCommands() {
 
   // For each OutputSection that needs a VA fabricate an OutputSectionCommand
   // with an InputSectionDescription describing the InputSections
   for (OutputSection *Sec : OutputSections) {
     auto *OSCmd = createOutputSectionCommand(Sec->Name, "<internal>");
     OSCmd->Sec = Sec;
     SecToCommand[Sec] = OSCmd;
 
-    // Prefer user supplied address over additional alignment constraint
-    auto I = Config->SectionStartMap.find(Sec->Name);
-    if (I != Config->SectionStartMap.end())
-      OSCmd->AddrExpr = [=] { return I->second; };
-
     Commands.push_back(OSCmd);
     if (Sec->Sections.size()) {
       auto *ISD = make<InputSectionDescription>("");
       OSCmd->Commands.push_back(ISD);
       for (InputSection *ISec : Sec->Sections) {
         ISD->Sections.push_back(ISec);
         ISec->Assigned = true;
       }

ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::finalizeSections() {
   sortSections();
 
   // Now that we have the final list, create a list of all the
   // OutputSectionCommands for convenience.
   for (BaseCommand *Base : Script->Opt.Commands)
     if (auto *Cmd = dyn_cast<OutputSectionCommand>(Base))
       OutputSectionCommands.push_back(Cmd);
 
+  // Prefer command line supplied address over other constraints.
+  for (OutputSectionCommand *Cmd : OutputSectionCommands) {
+    auto I = Config->SectionStartMap.find(Cmd->Name);
+    if (I != Config->SectionStartMap.end())
+      Cmd->AddrExpr = [=] { return I->second; };
+  }
+
   // This is a bit of a hack. A value of 0 means undef, so we set it
   // to 1 t make __ehdr_start defined. The section number is not
   // particularly relevant.
   Out::ElfHeader->SectionIndex = 1;
 
   unsigned I = 1;
   for (OutputSectionCommand *Cmd : OutputSectionCommands) {
     OutputSection *Sec = Cmd->Sec;

test/ELF/linkerscript/ttext-script.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: echo "SECTIONS { .text 0x200000 : { *(.text) } }" > %t.script
+# RUN: ld.lld -T %t.script -Ttext 0x100000 %t.o -o %t
+# RUN: llvm-readobj --elf-output-style=GNU -s  %t | FileCheck %s
+
+# CHECK: .text             PROGBITS        0000000000100000
+
+.global _start
+_start:
+nop