[Bug ld/31688] New: ld: Add SECTION_CLASS to allow separate section matching and referring

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=31688

            Bug ID: 31688
           Summary: ld: Add SECTION_CLASS to allow separate section
                    matching and referring
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

An input section description file_pattern(section_pattern) couples two
operations:

* "defining": match a subset of sections that have been been matched before
* "referring": assign them to the current output section

Roland McGrath has an idea that we can have a new syntax to separate the two
operations.
https://inbox.sourceware.org/binutils/CAB=4xhqKiXyFSm1nqrBFoJJ6VD3tvyVk-EHb2oxM+64hF9XUxg@mail.gmail.com/
(https://sourceware.org/pipermail/binutils/2024-February/132344.html)

    SECTIONS {
      /* Traditional output section clause: */
      .output1 { *(.input1) }

      /* New syntax that is not an output section clause: */
      SECTION_CLASS(class1) { *(.input2) }

      /* Output section clause referring to SECTION_CLASS: */
      .output2 {
        *(.input3) /* normal section wildcard */

        SECTION_CLASS(class1) /* reference to previously defined class */

        *(.input4) /* normal section wildcard */
      }

      .output3 {
        SECTION_CLASS(class1) /* reference to remainder of class not in
.output2 */
      }

      .output4 {
        /* reference to remainder not in .output[23], sorting applied to them
*/
        SORT_BY_NAME(SECTION_CLASS(class1))
      }

      /* This cannot match anything that went into a SECTION_CLASS and orphans
         placement does not apply to them so it's an error if any
         SECTION_CLASS-matched input section has not been assigned to a
         previous output section. */
      .output5 { *(*) }
    }

This will also be useful to tidy the .text section, whose description is
currently:

  .text           :
  {
    *(.text.unlikely .text.*_unlikely .text.unlikely.*)
    *(.text.exit .text.exit.*)
    *(.text.startup .text.startup.*)
    *(.text.hot .text.hot.*)
    *(SORT(.text.sorted.*))
    *(.text .stub .text.* .gnu.linkonce.t.*)
    /* .gnu.warning sections are handled specially by elf.em.  */
    *(.gnu.warning)
  }

We have to place .text.unlikely/.text.startup/.text.hot before .text.* as
otherwise .text.* would match all text sections and render .textXXXX patterns
no-ops.
With SECTION_CLASS, if a user wants to make
.text.unlikely/.text.startup/.text.hot output sections (gold/ld.lld -z
keep-text-section-prefix; allow a program to selectively place sections into
hugepages) and if they want to place .text.startup/.text.hot after .text, they
can use:

  SECTION_CLASS(text_startup) { *(.text.startup .text.startup.*) }
  SECTION_CLASS(text_hot) { *(.text.hot .text.hot.*) }
  .text : { *(.text .text.*) }
  .text.startup : { SECTION_CLASS(text_startup) }
  .text.hot : { SECTION_CLASS(text_hot) }

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