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

[Derek Schuff]

- add comments and tighten test


http://reviews.llvm.org/D9131

Files:
  lib/MC/MCELFStreamer.cpp
  test/MC/X86/AlignedBundling/section-alignment.s

Index: lib/MC/MCELFStreamer.cpp
===================================================================
--- lib/MC/MCELFStreamer.cpp
+++ lib/MC/MCELFStreamer.cpp
@@ -131,13 +131,25 @@
   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();
   if (CurSection && CurSection->isBundleLocked())
     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::FinishImpl() {
+  // Ensure the last section gets aligned if necessary.
+  setSectionAlignmentForBundling(getAssembler(), getCurrentSectionData());
+
   EmitFrames(nullptr);
 
   Flush();
Index: test/MC/X86/AlignedBundling/section-alignment.s
===================================================================
--- /dev/null
+++ 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

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