Handle section header flags redefinitions similar to GAS

[christof]

Summary

A .section directive defining an already existing section has now the following behavior:

• target specific flags (that is OS or PROC specific) are added to the existing section
• Any other flag is ignored and a warning is emitted

This makes the integrated assembler behave like GNU as version 2.27.

[christof]

Summary from email conversation:

• GAS redefinitions is considered bad style
• Removing the pre-constructed .text/.bss/.data/... sections from MC is a possible better way, although Rafael did not come back if he likes that approach.

I'm abandoning this patch, as this mimics GAS too closely. And the patch at https://reviews.llvm.org/rL296180 disables redefinitions.

lib/MC/MCParser/ELFAsmParser.cpp

if (UseLastGroup) {
     if (const MCSectionELF *Section =
             cast_or_null<MCSectionELF>(CurrentSection.first))
       if (const MCSymbol *Group = Section->getGroup()) {
         GroupName = Group->getName();
         Flags |= ELF::SHF_GROUP;
       }
   }
 
-  MCSection *ELFSection = getContext().getELFSection(
+  MCSectionELF *ELFSection = getContext().getELFSection(
       SectionName, Type, Flags, Size, GroupName, UniqueID, Associated);
+  // Sections are not identified by flags. Check if there is a mismatch in the
+  // flags requested and the flags in the section returned. If the mismatch is
+  // in OS or CPU specific flags, just add them. For any other mismatch warn
+  // the user that we are ignoring the new flags.
+  const unsigned overrideMask = (ELF::SHF_MASKOS | ELF::SHF_MASKPROC);
+  unsigned curFlags = ELFSection->getFlags();
+  curFlags |= Flags & overrideMask;
+  Flags |= curFlags & overrideMask;
+  ELFSection->setFlags(curFlags);
+  if (curFlags != Flags)
+    Warning(loc, "ignoring override of non-target sepecific section flags");
+
   getStreamer().SwitchSection(ELFSection, Subsection);
 
   if (getContext().getGenDwarfForAssembly()) {
     bool InsertResult = getContext().addGenDwarfSection(ELFSection);
     if (InsertResult) {
       if (getContext().getDwarfVersion() <= 2)
         Warning(loc, "DWARF2 only supports one section per compilation unit");
 

test/MC/ELF/section-override-flags.s

+// RUN: llvm-mc -filetype=obj -triple arm-pc-linux-gnu %s -o %t 2>&1 | FileCheck -check-prefix=DIAG %s
+// RUN: llvm-readobj -s %t | FileCheck %s
+
+// REQUIRES: arm-registered-target
+
+// Test we handle overriding OS and PROC specific flags on the builtin .text section
+.section .text,"0x200006",%progbits
+.section .text,"0x40000006",%progbits
+
+// CHECK:      Section {
+// CHECK:        Name: .text
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x40200006)
+// CHECK-NEXT:     SHF_ALLOC (0x2)
+// CHECK-NEXT:     SHF_EXECINSTR (0x4)
+// CHECK-NEXT:   ]
+
+//Attempt to override flags that are not target specific should be ignored
+.section foo,"ax",%progbits
+.section foo,"0x7",%progbits
+.section foo,"0x10000007",%progbits
+
+// DIAG: warning: ignoring override of non-target sepecific section flags
+// DIAG: warning: ignoring override of non-target sepecific section flags
+
+// CHECK:      Section {
+// CHECK:        Name: foo
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x10000006)
+// CHECK-NEXT:     SHF_ALLOC (0x2)
+// CHECK-NEXT:     SHF_EXECINSTR (0x4)
+// CHECK-NEXT:   ]
+
+
+// We want to see this warning the exact number times specified
+// DIAG-NOT: warning: ignoring override of non-target sepecific section flags