Revert [MC][ELF] Emit separate unique sections for different flags

Commit Message from @MaskRay:

Rust has a fragile embed-bitcode implementation
(https://github.com/rust-lang/rust/blob/bddb59cf07efcf6e606f16b87f85e3ecd2c1ca69/compiler/rustc_codegen_llvm/src/back/write.rs#L970-L1017)
which relied on the previous LLVM MC behavior.  Rust's LLVM fork
has carried a revert. This commit made the similar revert to help
distributions since they would otherwise probably carry a similar patch
(as they ship rust linked against system LLVM).

Fixes https://bugs.llvm.org/show_bug.cgi?id=51207.

Differential Revision: https://reviews.llvm.org/D107216

Author: nikic

llvm/include/llvm/MC/MCContext.h

@@ -374,17 +374,17 @@ namespace llvm {
       bool operator<(const ELFEntrySizeKey &Other) const {
         if (SectionName != Other.SectionName)
           return SectionName < Other.SectionName;
-        if (Flags != Other.Flags)
-          return Flags < Other.Flags;
+        if ((Flags & ELF::SHF_STRINGS) != (Other.Flags & ELF::SHF_STRINGS))
+          return Other.Flags & ELF::SHF_STRINGS;
         return EntrySize < Other.EntrySize;
       }
     };
 
-    // Symbols must be assigned to a section with a compatible entry size and
-    // flags. This map is used to assign unique IDs to sections to distinguish
-    // between sections with identical names but incompatible entry sizes and/or
-    // flags. This can occur when a symbol is explicitly assigned to a section,
-    // e.g. via __attribute__((section("myname"))).
+    // Symbols must be assigned to a section with a compatible entry
+    // size. This map is used to assign unique IDs to sections to
+    // distinguish between sections with identical names but incompatible entry
+    // sizes. This can occur when a symbol is explicitly assigned to a
+    // section, e.g. via __attribute__((section("myname"))).
     std::map<ELFEntrySizeKey, unsigned> ELFEntrySizeMap;
 
     // This set is used to record the generic mergeable section names seen.
@@ -592,8 +592,6 @@ namespace llvm {
 
     bool isELFGenericMergeableSection(StringRef Name);
 
-    /// Return the unique ID of the section with the given name, flags and entry
-    /// size, if it exists.
     Optional<unsigned> getELFUniqueIDForEntsize(StringRef SectionName,
                                                 unsigned Flags,
                                                 unsigned EntrySize);

llvm/lib/MC/MCContext.cpp

@@ -586,7 +586,7 @@ void MCContext::recordELFMergeableSectionInfo(StringRef SectionName,
                                               unsigned Flags, unsigned UniqueID,
                                               unsigned EntrySize) {
   bool IsMergeable = Flags & ELF::SHF_MERGE;
-  if (UniqueID == GenericSectionID)
+  if (IsMergeable && (UniqueID == GenericSectionID))
     ELFSeenGenericMergeableSections.insert(SectionName);
 
   // For mergeable sections or non-mergeable sections with a generic mergeable

llvm/test/CodeGen/Mips/gpopt-explict-section.ll

@@ -7,7 +7,7 @@
 ; small data section. Also test that explicitly placing something in the small
 ; data section uses %gp_rel addressing mode.
 
-@a = constant [2 x i32] zeroinitializer, section ".rodata", align 4
+@a = global [2 x i32] zeroinitializer, section ".rodata", align 4
 @b = global [4 x i32] zeroinitializer, section ".sdata", align 4
 @c = global [4 x i32] zeroinitializer, section ".sbss", align 4