[MC][ELF] Accept abbreviated form with sh_flags and sh_entsize

D73999 / commit 75af9da
added for LLVM 11 a check that sh_flags and sh_entsize (and sh_type)
changes are an error, in line with GNU assembler.

However, GNU assembler accepts and GCC generates an abbreviated form:
while the first .section contains the flags and entsize, subsequent
sections simply contain the name without repeating entsize or flags.

Do likewise for better compatibility.

See https://bugs.llvm.org/show_bug.cgi?id=48201

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D92052

(cherry picked from commit 1deff40)

llvm/lib/MC/MCParser/ELFAsmParser.cpp

@@ -644,10 +644,13 @@ bool ELFAsmParser::ParseSectionArguments(bool IsPush, SMLoc loc) {
       !(SectionName == ".eh_frame" && Type == ELF::SHT_PROGBITS))
     Error(loc, "changed section type for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getType()));
-  if (Section->getFlags() != Flags)
+  // Check that flags are used consistently. However, the GNU assembler permits
+  // to leave out in subsequent uses of the same sections; for compatibility,
+  // do likewise.
+  if ((Flags || Size || !TypeName.empty()) && Section->getFlags() != Flags)
     Error(loc, "changed section flags for " + SectionName + ", expected: 0x" +
                    utohexstr(Section->getFlags()));
-  if (Section->getEntrySize() != Size)
+  if ((Flags || Size || !TypeName.empty()) && Section->getEntrySize() != Size)
     Error(loc, "changed section entsize for " + SectionName +
                    ", expected: " + Twine(Section->getEntrySize()));
 

llvm/test/MC/ELF/section-flags-changed.s

@@ -9,4 +9,7 @@ foo:
 # CHECK: {{.*}}.s:[[# @LINE+1]]:1: error: changed section flags for .foo, expected: 0x6
 .pushsection .foo,"a",@progbits
 
+# CHECK: {{.*}}.s:[[# @LINE+1]]:1: error: changed section flags for .foo, expected: 0x6
+.section .foo,"",@progbits
+
 .pushsection .foo,"ax",@progbits

llvm/test/MC/ELF/section-omitted-attributes.s

@@ -0,0 +1,11 @@
+# RUN: llvm-mc -triple=x86_64 %s -o - | FileCheck %s
+
+# If section flags and other attributes are omitted, don't error.
+
+# CHECK: .section        .foo,"aM",@progbits,1
+
+.section .foo,"aM",@progbits,1
+
+.section .foo
+
+.pushsection .foo