Fix SUBALIGN linker script feature

This commit fixes the SUBALIGN linker script feature. SUBALIGN feature
is used to modify the alignment of all input sections present in an
output section. Until now, we were only changing the alignment of the
first fragment of each rule of the output sections with SUBALIGN
specified. This commit fixes the SUBALIGN behavior.

SUBALIGN affects the alignment of the first fragment as is seen by the linker
when traversing the output sections for the input sections
that may contain more than 1 fragment such as .eh_frame, PLT / GOT sections,
and .comment section.

Resolves #343

Signed-off-by: Parth Arora <partaror@qti.qualcomm.com>

Author: parth-07

include/eld/Object/ObjectLinker.h

@@ -312,6 +312,8 @@ class ObjectLinker {
 
   bool initializeOutputSectionsAndRunPlugin();
 
+  void applySubAlign();
+
   // Get Plugin list for Relocation registration callback.
   LinkerScript::PluginVectorT getLinkerPluginWithLinkerConfigs();
 

lib/Object/ObjectLinker.cpp

@@ -80,6 +80,7 @@
 #include <chrono>
 #include <mutex>
 #include <sstream>
+#include <unordered_set>
 
 using namespace llvm;
 using namespace eld;
@@ -1248,6 +1249,9 @@ bool ObjectLinker::mergeSections() {
   {
     eld::RegisterTimer T("Create Output Section", "Merge Sections",
                          ThisConfig.options().printTimingStats());
+    // Prepass: apply SUBALIGN to first fragments per input section per output
+    // section serially to avoid races in parallel output creation.
+    applySubAlign();
     std::vector<OutputSectionEntry *> OutSections;
     for (Out = OutBegin; Out != OutEnd; ++Out) {
       OutSections.push_back(*Out);
@@ -1291,6 +1295,33 @@ bool ObjectLinker::initializeOutputSectionsAndRunPlugin() {
   return runOutputSectionIteratorPlugin();
 }
 
+void ObjectLinker::applySubAlign() {
+  std::unordered_set<ELFSection *> seen;
+
+  for (auto *O : ThisModule->getScript().sectionMap()) {
+    auto &Prolog = O->prolog();
+    std::optional<uint64_t> optSubAlign;
+    if (Prolog.hasSubAlign()) {
+      Prolog.subAlign().eval();
+      Prolog.subAlign().commit();
+      optSubAlign = Prolog.subAlign().result();
+    }
+
+    for (RuleContainer *R : *O) {
+      ELFSection *InSect = R->getSection();
+      for (Fragment *F : InSect->getFragmentList()) {
+        ELFSection *owningSect = F->getOwningSection();
+        if (owningSect && seen.insert(owningSect).second && optSubAlign) {
+          F->setAlignment(optSubAlign.value());
+          InSect->setAddrAlign(
+              std::max(static_cast<uint64_t>(InSect->getAddrAlign()),
+                       optSubAlign.value()));
+        }
+      }
+    }
+  }
+}
+
 void ObjectLinker::assignOffset(OutputSectionEntry *Out) {
   int64_t O = 0;
   const ELFSection *OutSection = Out->getSection();

test/Common/standalone/SubAlign/Inputs/script.2.t

@@ -0,0 +1,14 @@
+PHDRS {
+  A PT_LOAD;
+}
+
+SECTIONS {
+  .a : SUBALIGN(64) {
+   . = . + 1;
+    *(.text*)
+  }:A
+  .b : SUBALIGN(64) {
+   . = . + 1;
+  }:A
+  /DISCARD/ : { *(.ARM.exidx*) }
+}

test/Common/standalone/SubAlign/SubAlign.test

@@ -4,8 +4,11 @@
 #END_COMMENT
 #START_TEST
 RUN: %clang %clangopts -c %p/Inputs/1.c -o %t1.1.o -ffunction-sections
-RUN: %link -MapStyle txt %linkopts %t1.1.o -T %p/Inputs/script.t -o %t1.1.o -Map %t2.map
-RUN: %filecheck %s < %t2.map
+RUN: %link -MapStyle txt %linkopts %t1.1.o -T %p/Inputs/script.t -o %t1.1.out -Map %t1.1.map.txt
+RUN: %filecheck %s < %t1.1.map.txt
+RUN: %link -MapStyle txt %linkopts %t1.1.o -T %p/Inputs/script.2.t -o %t1.2.out -Map %t1.2.map.txt
+RUN: %filecheck %s < %t1.1.map.txt
+RUN: %filecheck %s < %t1.2.map.txt
 #CHECK: .a {{.*}} Alignment: 0x40
 #CHECK: .text.foo	0x40
 #CHECK: .text.bar	0x80