[all-commits] [llvm/llvm-project] 660552: [TargetLoweringObjectFileImpl] Produce .text.hot. ...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 66055230bf6673b76e7330fef76d752a1ea8638e
      https://github.com/llvm/llvm-project/commit/66055230bf6673b76e7330fef76d752a1ea8638e
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-05-12 (Tue, 12 May 2020)

  Changed paths:
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    M llvm/test/Transforms/CodeGenPrepare/X86/section.ll

  Log Message:
  -----------
  [TargetLoweringObjectFileImpl] Produce .text.hot. instead of .text.hot for -fno-unique-section-names

GNU ld's internal linker script uses (https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=add44f8d5c5c05e08b11e033127a744d61c26aee)

  .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)
  }

Because `*(.text.exit .text.exit.*)` is ordered before `*(.text .text.*)`, in a -ffunction-sections build, the C library function `exit` will be placed before other functions.
gold's `-z keep-text-section-prefix` has the same problem.

In lld, `-z keep-text-section-prefix` recognizes `.text.{exit,hot,startup,unlikely,unknown}.*`, but not `.text.{exit,hot,startup,unlikely,unknown}`, to avoid the strange placement problem.

In -fno-function-sections or -fno-unique-section-names mode, a function whose `function_section_prefix` is set to `.exit"`
will go to the output section `.text` instead of `.text.exit` when linked by lld.
To address the problem, append a dot to become `.text.exit.`

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D79600