[ELF] - Linkerscript: implemented SUBALIGN() command.

You can force input section alignment within an output section by using SUBALIGN. The
value specified overrides any alignment given by input sections, whether larger or smaller.

SUBALIGN is used in many projects in the wild.

Differential revision: https://reviews.llvm.org/D23063

llvm-svn: 279256

lld/ELF/LinkerScript.cpp

@@ -283,9 +283,14 @@ void LinkerScript<ELFT>::createSections(OutputSectionFactory<ELFT> &Factory) {
       std::tie(OutSec, IsNew) = Factory.create(Head, Cmd->Name);
       if (IsNew)
         OutputSections->push_back(OutSec);
-      for (InputSectionBase<ELFT> *Sec : V)
+
+      uint32_t Subalign = Cmd->SubalignExpr ? Cmd->SubalignExpr(0) : 0;
+      for (InputSectionBase<ELFT> *Sec : V) {
+        if (Subalign)
+          Sec->Alignment = Subalign;
         if (!Sec->OutSec)
           OutSec->addSection(Sec);
+      }
     }
   }
 
@@ -937,6 +942,8 @@ ScriptParser::readOutputSectionDescription(StringRef OutSec) {
     Cmd->LmaExpr = readParenExpr();
   if (skip("ALIGN"))
     Cmd->AlignExpr = readParenExpr();
+  if (skip("SUBALIGN"))
+    Cmd->SubalignExpr = readParenExpr();
 
   // Parse constraints.
   if (skip("ONLY_IF_RO"))

lld/ELF/LinkerScript.h

@@ -81,6 +81,7 @@ struct OutputSectionCommand : BaseCommand {
   Expr AddrExpr;
   Expr AlignExpr;
   Expr LmaExpr;
+  Expr SubalignExpr;
   std::vector<std::unique_ptr<BaseCommand>> Commands;
   std::vector<StringRef> Phdrs;
   std::vector<uint8_t> Filler;

lld/test/ELF/linkerscript/linkerscript-subalign.s

@@ -0,0 +1,43 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+
+# RUN: echo "SECTIONS { .aaa : { *(.aaa.*) } }" > %t1.script
+# RUN: ld.lld -o %t1 --script %t1.script %t1.o
+# RUN: llvm-objdump -s %t1 | FileCheck -check-prefix=NOALIGN %s
+# NOALIGN:      Contents of section .aaa:
+# NOALIGN-NEXT:   01000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   00000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   02000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   00000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   03000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   00000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   00000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   00000000 00000000 00000000 00000000
+# NOALIGN-NEXT:   04000000 00000000
+
+# RUN: echo "SECTIONS { .aaa : SUBALIGN(1) { *(.aaa.*) } }" > %t2.script
+# RUN: ld.lld -o %t2 --script %t2.script %t1.o
+# RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=SUBALIGN %s
+# SUBALIGN: Contents of section .aaa:
+# SUBALIGN:   01000000 00000000 02000000 00000000
+# SUBALIGN:   03000000 00000000 04000000 00000000
+
+.global _start
+_start:
+ nop
+
+.section .aaa.1, "a"
+.align 16
+.quad 1
+
+.section .aaa.2, "a"
+.align 32
+.quad 2
+
+.section .aaa.3, "a"
+.align 64
+.quad 3
+
+.section .aaa.4, "a"
+.align 128
+.quad 4