[lld][ELF] Do not emit warning for NOLOAD output sections

[leonardchan]

Summary

Much of NOLOAD's intended use is to explicitly change the type of an output section, so we shouldn't flag these as warnings.

[peter.smith]

Thanks very much for posting the alternative patch. One small suggestion to update the comment.

I had been thinking about whether it was worth distinguishing between (NOLOAD) and (TYPE=SHT_NOBITS), i.e. error for the latter but do nothing for the former. However GNU ld does not diagnose either case, and as TYPE=SHT_NOBITS is likely to be an expert level feature, it is probably best leaving as is.

Best to wait a bit to see if MaskRay has any comments as well.

lld/ELF/OutputSections.cpp

118–119

I think the comment is worth updating. For example

The (NOLOAD) changes the section type to SHT_NOBITS, the intention is that the contents at that address is provided by some other means. Some projects (e.g. https://github.com/ClangBuiltLinux/linux/issues/1597) rely on the behavior. Other types get an error.

[MaskRay]

lld/test/ELF/linkerscript/noload.s

25

Check that there is no warning. An exact message can easily get stale.

lld/ELF/OutputSections.cpp

 
 // Update fields (type, flags, alignment, etc) according to the InputSection
 // isec. Also check whether the InputSection flags and type are consistent with
 // other InputSections.
 void OutputSection::commitSection(InputSection *isec) {
   if (LLVM_UNLIKELY(type != isec->type)) {
     if (hasInputSections || typeIsSet) {
       if (typeIsSet || !canMergeToProgbits(type) ||
           !canMergeToProgbits(isec->type)) {
-        // Changing the type of a (NOLOAD) section is fishy, but some projects
-        // (e.g. https://github.com/ClangBuiltLinux/linux/issues/1597)
-        // traditionally rely on the behavior. Issue a warning to not break
-        // them. Other types get an error.
-        auto diagnose = type == SHT_NOBITS ? warn : errorOrWarn;
-        diagnose("section type mismatch for " + isec->name + "\n>>> " +
+        // The (NOLOAD) changes the section type to SHT_NOBITS, the intention is
+        // that the contents at that address is provided by some other means.
+        // Some projects (e.g.
+        // https://github.com/ClangBuiltLinux/linux/issues/1597) rely on the
+        // behavior. Other types get an error.
+        if (type != SHT_NOBITS) {
+          errorOrWarn("section type mismatch for " + isec->name + "\n>>> " +
                       toString(isec) + ": " +
                       getELFSectionTypeName(config->emachine, isec->type) +
                       "\n>>> output section " + name + ": " +
                       getELFSectionTypeName(config->emachine, type));
         }
+      }
       if (!typeIsSet)
         type = SHT_PROGBITS;
     } else {
       type = isec->type;
     }
   }
   if (!hasInputSections) {
     // If IS is the first section to be added to this section,

lld/test/ELF/linkerscript/noload.s

 # CHECK-NEXT: LOAD 0x001000 0x0000000000000000 0x0000000000000000
 # CHECK-NEXT: LOAD 0x001000 0x0000000000020000 0x0000000000020000
 
 # CHECK:      00 .data_noload_a .data_noload_b .no_input_sec_noload {{$}}
 # CHECK:      01 .text {{$}}
 
 ## The output SHT_PROBITS is contrary to the user expectation of SHT_NOBITS.
 ## Issue a warning. See https://github.com/ClangBuiltLinux/linux/issues/1597
-# RUN: ld.lld --script %t/lds %t.o %t/mismatch.o -o %t/out 2>&1 | FileCheck %s --check-prefix=WARN
+# RUN: ld.lld --script %t/lds %t.o %t/mismatch.o -o %t/out 2>&1 | FileCheck %s --check-prefix=WARN --allow-empty
 # RUN: llvm-readelf -S -l %t/out | FileCheck %s --check-prefix=CHECK2
 
-# WARN:   warning: section type mismatch for .data_noload_a
+# WARN-NOT:   warning:
 # CHECK2:      Name                 Type     Address          Off               Size
 # CHECK2:      .data_noload_a       NOBITS   0000000000000000 [[OFF:[0-9a-f]+]] 001001
 
 #--- asm
 .section .text,"ax",@progbits
   nop
 
 .section .data_noload_a,"aw",@nobits
 .zero 4096
 
 .section .data_noload_b,"aw",@nobits
 .zero 4096
 
 #--- mismatch.s
 .section .data_noload_a,"aw",@progbits
 .byte 1
 
 #--- lds
 SECTIONS {
   .data_noload_a (NOLOAD) : { *(.data_noload_a) }
   .data_noload_b (0x10000) (NOLOAD) : { *(.data_noload_b) }
   .no_input_sec_noload (NOLOAD) : { . += 1; }
-  .text (0x20000) : { *(.text) }
+  .text (0x20000) : {
+    *(.text);
+    _start = .;
+  }
 }