[MC] When using bundle aligment, align sections to bundle size

Summary:
Bundle aligment requires that the functions always start at an aligned address.
Usually this is ensured by the compiler, but assembly code does not always
begin with a .align directive.

This change ensures that sections get the correct alignment if they contain
any instructions and bundling is enabled. (It also makes LLVM match the
behavior of GNU as).

Differential Revision: http://reviews.llvm.org/D9131

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235365 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/MC/MCELFStreamer.cpp b/lib/MC/MCELFStreamer.cpp
index aa0539077255c53d2a1024dc1b48df3eacaa0f72..24b5681e55a308c00af6cf5508b4f6e0eade42e6 100644 (file)
--- a/lib/MC/MCELFStreamer.cpp
+++ b/lib/MC/MCELFStreamer.cpp
@@ -131,6 +131,16 @@ void MCELFStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
   llvm_unreachable("invalid assembler flag!");
 }
 
+// If bundle aligment is used and there are any instructions in the section, it
+// needs to be aligned to at least the bundle size.
+static void setSectionAlignmentForBundling(
+    const MCAssembler &Assembler, MCSectionData *Section) {
+  if (Assembler.isBundlingEnabled() && Section &&
+      Section->hasInstructions() &&
+      Section->getAlignment() < Assembler.getBundleAlignSize())
+    Section->setAlignment(Assembler.getBundleAlignSize());
+}
+
 void MCELFStreamer::ChangeSection(const MCSection *Section,
                                   const MCExpr *Subsection) {
   MCSectionData *CurSection = getCurrentSectionData();
@@ -138,6 +148,8 @@ void MCELFStreamer::ChangeSection(const MCSection *Section,
     report_fatal_error("Unterminated .bundle_lock when changing a section");
 
   MCAssembler &Asm = getAssembler();
+  // Ensure the previous section gets aligned if necessary.
+  setSectionAlignmentForBundling(Asm, CurSection);
   auto *SectionELF = static_cast<const MCSectionELF *>(Section);
   const MCSymbol *Grp = SectionELF->getGroup();
   if (Grp)
@@ -639,6 +651,9 @@ void MCELFStreamer::Flush() {
 }
 
 void MCELFStreamer::FinishImpl() {
+  // Ensure the last section gets aligned if necessary.
+  setSectionAlignmentForBundling(getAssembler(), getCurrentSectionData());
+
   EmitFrames(nullptr);
 
   Flush();

diff --git a/test/MC/X86/AlignedBundling/section-alignment.s b/test/MC/X86/AlignedBundling/section-alignment.s
new file mode 100644 (file)
index 0000000..c40432c
--- /dev/null
+++ b/test/MC/X86/AlignedBundling/section-alignment.s
@@ -0,0 +1,21 @@
+# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - \
+# RUN:   | llvm-readobj -sections | FileCheck %s
+
+# Test that bundle-aligned sections with instructions are aligned
+
+  .bundle_align_mode 5
+# CHECK: Sections
+# Check that the empty .text section has the default alignment
+# CHECK-LABEL: Name: .text
+# CHECK-NOT: Name
+# CHECK: AddressAlignment: 4
+
+  .section text1, "x"
+  imull $17, %ebx, %ebp
+# CHECK-LABEL: Name: text1
+# CHECK: AddressAlignment: 32
+
+  .section text2, "x"
+  imull $17, %ebx, %ebp
+# CHECK-LABEL: Name: text2
+# CHECK: AddressAlignment: 32