Add support for #pragma clang section

This patch provides a means to specify section-names for global variables,
functions and static variables, using #pragma directives.
This feature is only defined to work sensibly for ELF targets.
One can specify section names as:
#pragma clang section bss="myBSS" data="myData" rodata="myRodata" text="myText"
One can "unspecify" a section name with empty string e.g.
#pragma clang section bss="" data="" text="" rodata=""

Reviewers: Roger Ferrer, Jonathan Roelofs, Reid Kleckner
Differential Revision: https://reviews.llvm.org/D33413

llvm-svn: 304704

3 files changed, 35 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/GlobalMerge.cpp b/llvm/lib/CodeGen/GlobalMerge.cpp
index 3603f9b7ed9..c6ca49ce24d 100644
--- a/llvm/lib/CodeGen/GlobalMerge.cpp
+++ b/llvm/lib/CodeGen/GlobalMerge.cpp
@@ -553,7 +553,8 @@ bool GlobalMerge::doInitialization(Module &M) {
   // Grab all non-const globals.
   for (auto &GV : M.globals()) {
     // Merge is safe for "normal" internal or external globals only
-    if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
+    if (GV.isDeclaration() || GV.isThreadLocal() ||
+        GV.hasSection() || GV.hasImplicitSection())
       continue;
 
     // It's not safe to merge globals that may be preempted
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 1d232c71d82..3ba4a3a2926 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -248,6 +248,25 @@ MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   StringRef SectionName = GO->getSection();
 
+  // Check if '#pragma clang section' name is applicable.
+  // Note that pragma directive overrides -ffunction-section, -fdata-section
+  // and so section name is exactly as user specified and not uniqued.
+  const GlobalVariable *GV = dyn_cast<GlobalVariable>(GO);
+  if (GV && GV->hasImplicitSection()) {
+    auto Attrs = GV->getAttributes();
+    if (Attrs.hasAttribute("bss-section") && Kind.isBSS()) {
+      SectionName = Attrs.getAttribute("bss-section").getValueAsString();
+    } else if (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()) {
+      SectionName = Attrs.getAttribute("rodata-section").getValueAsString();
+    } else if (Attrs.hasAttribute("data-section") && Kind.isData()) {
+      SectionName = Attrs.getAttribute("data-section").getValueAsString();
+    }
+  }
+  const Function *F = dyn_cast<Function>(GO);
+  if (F && F->hasFnAttribute("implicit-section-name")) {
+    SectionName = F->getFnAttribute("implicit-section-name").getValueAsString();
+  }
+
   // Infer section flags from the section name if we can.
   Kind = getELFKindForNamedSection(SectionName, Kind);
 
diff --git a/llvm/lib/Target/TargetLoweringObjectFile.cpp b/llvm/lib/Target/TargetLoweringObjectFile.cpp
index 91cc97e38b3..293a62007af 100644
--- a/llvm/lib/Target/TargetLoweringObjectFile.cpp
+++ b/llvm/lib/Target/TargetLoweringObjectFile.cpp
@@ -240,6 +240,20 @@ MCSection *TargetLoweringObjectFile::SectionForGlobal(
   if (GO->hasSection())
     return getExplicitSectionGlobal(GO, Kind, TM);
 
+  if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
+    auto Attrs = GVar->getAttributes();
+    if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
+        (Attrs.hasAttribute("data-section") && Kind.isData()) ||
+        (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()))  {
+       return getExplicitSectionGlobal(GO, Kind, TM);
+    }
+  }
+
+  if (auto *F = dyn_cast<Function>(GO)) {
+    if (F->hasFnAttribute("implicit-section-name"))
+      return getExplicitSectionGlobal(GO, Kind, TM);
+  }
+
   // Use default section depending on the 'type' of global
   return SelectSectionForGlobal(GO, Kind, TM);
 }