Print numeric section flag for OS/processor specific bits

[prakhar]

Summary

When outputting assembly, any OS or processor-specific flags cannot be
represented by symbolic letters, so these are lost in the generated
assembly. This patch therefore prints out the flags in numeric form if
any of these bits are set. This is compatible with how GAS handles
unknown flags, and allows the generation of assembly that can be read by
GAS.

[rengolin]

Wait, but we do know those flags, so it doesn't make sense to handle as unknown just because GAS does it.

We should support reading both numeric and textual versions, but it makes no sense to emit unknown.

[prakhar]

None of the other target-specific flags have textual representations though, and the y symbol for ARM pure code was a proposal that was ultimately rejected. Hence, if these flags are present in the code, their information will be lost in the assembler output as they do not have a textual representation. It is therefore safer to default to a numeric representation that is already understood by both toolchains if these flags are present.

[prakhar]

Ping for this review

[prakhar]

@rengolin Ping again for this review

[christof]

Hi Phrakar. I think you mean that there are no processor or OS specific flags with textual representation. But yes, SHF_ARM_PURECODE is the first of its kind. Makes me wonder if we are better of not having a textual representation at all for the flag.

lib/MC/MCSectionELF.cpp

95

I believe that the right style is to give the whole if-else has curly braces ("{", "}") or none. Since you need it at the else clause, you have to add it here as well.

[prakhar]

Style and formatting fix

[prakhar]

Yes, since no other OS/processor specific flag has a text equivalent, it might be better to not emit one for purecode at all. This is independent of the fact that these flags are being lost in the assembler output as LLVM currently does not emit them in numeric form.

[prakhar]

Abandoning as this is now being handled in https://reviews.llvm.org/D30169

lib/MC/MCSectionELF.cpp

 //===- lib/MC/MCSectionELF.cpp - ELF Code Section Representation ----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/Format.h"
 
 using namespace llvm;
 
 MCSectionELF::~MCSectionELF() {} // anchor.
 
 // Decides whether a '.section' directive
 // should be printed before the section name.
 bool MCSectionELF::ShouldOmitSectionDirective(StringRef Name,
if (Flags & ELF::SHF_EXCLUDE)
       OS << ",#exclude";
     if (Flags & ELF::SHF_TLS)
       OS << ",#tls";
     OS << '\n';
     return;
   }
 
   OS << ",\"";
+
+  // If the flag has OS or processor-specific bits set, print the whole numeric
+  // flag, otherwise use the symbolic representation
+  if (Flags & (ELF::SHF_MASKOS | ELF::SHF_MASKPROC)) {
+    OS << format_hex(Flags, 0);
+  } else {
     if (Flags & ELF::SHF_ALLOC)
       OS << 'a';
     if (Flags & ELF::SHF_EXCLUDE)
       OS << 'e';
     if (Flags & ELF::SHF_EXECINSTR)
       OS << 'x';
     if (Flags & ELF::SHF_GROUP)
       OS << 'G';
     if (Flags & ELF::SHF_WRITE)
       OS << 'w';
     if (Flags & ELF::SHF_MERGE)
       OS << 'M';
     if (Flags & ELF::SHF_STRINGS)
       OS << 'S';
     if (Flags & ELF::SHF_TLS)
       OS << 'T';
 
     // If there are target-specific flags, print them.
     if (Flags & ELF::XCORE_SHF_CP_SECTION)
       OS << 'c';
     if (Flags & ELF::XCORE_SHF_DP_SECTION)
       OS << 'd';
-  if (Flags & ELF::SHF_ARM_PURECODE)
-    OS << 'y';
+  }
 
   OS << '"';
 
   OS << ',';
 
   // If comment string is '@', e.g. as on ARM - use '%' instead
   if (MAI.getCommentString()[0] == '@')
     OS << '%';

test/CodeGen/ARM/execute-only-section.ll

 ; RUN: llc < %s -mtriple=thumbv7m -arm-execute-only %s -o - | FileCheck %s
 ; RUN: llc < %s -mtriple=thumbv8m.base -arm-execute-only %s -o - | FileCheck %s
 ; RUN: llc < %s -mtriple=thumbv8m.main -arm-execute-only %s -o - | FileCheck %s
 
-; CHECK:     .section .text,"axy",%progbits,unique,0
+; CHECK:     .section .text,"0x20000006",%progbits,unique,0
 ; CHECK-NOT: .section
 ; CHECK-NOT: .text
 ; CHECK:     .globl test_SectionForGlobal
 ; CHECK:     .type test_SectionForGlobal,%function
 define void @test_SectionForGlobal() {
 entry:
   ret void
 }
 
-; CHECK:     .section .test,"axy",%progbits
+; CHECK:     .section .test,"0x20000006",%progbits
 ; CHECK-NOT: .section
 ; CHECK-NOT: .text
 ; CHECK:     .globl test_ExplicitSectionForGlobal
 ; CHECK:     .type test_ExplicitSectionForGlobal,%function
 define void @test_ExplicitSectionForGlobal() section ".test" {
 entry:
   ret void
 }