lkml.org 
[lkml]   [2022]   [Oct]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    Subject[PATCH v3] kbuild: add -fno-discard-value-names to cmd_cc_ll_c
    From
    When debugging LLVM IR, it can be handy for clang to not discard value
    names used for local variables and parameters. Compare the generated IR.

    -fdiscard-value-names:
    define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
    %6 = alloca i64
    %7 = alloca %struct.poll_wqueues
    %8 = alloca [64 x i32]

    -fno-discard-value-names:
    define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
    ptr %end_time) {
    %expire.i = alloca i64
    %table.i = alloca %struct.poll_wqueues
    %stack_fds = alloca [64 x i32]

    The rule for generating human readable LLVM IR (.ll) is only useful as a
    debugging feature:

    $ make LLVM=1 fs/select.ll

    As Fangrui notes:
    A LLVM_ENABLE_ASSERTIONS=off build of Clang defaults to
    -fdiscard-value-names.

    A LLVM_ENABLE_ASSERTIONS=on build of Clang defaults to
    -fno-discard-value-names.

    Explicitly enable -fno-discard-value-names so that the IR always contains
    value names regardless of whether assertions were enabled or not.
    Assertions generally are not enabled in releases of clang packaged by
    distributions.

    Link: https://github.com/ClangBuiltLinux/linux/issues/1467
    Reviewed-by: Nathan Chancellor <nathan@kernel.org>
    Reviewed-by: Fangrui Song <maskray@google.com>
    Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    ---
    Changes v2 -> v3:
    * Fix typo in commit message
    "enable -fdiscard-value-names" -> "enable -fno-discard-value-names"

    Changes v1 -> v2:
    * Only update commit message:
    * Pick up Nathan & Fangrui's RB tags.
    * Add link (I forgot I filed a bug about this a while ago!).
    * Add note from Fangrui.
    * Add final paragraph.
    scripts/Makefile.build | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/scripts/Makefile.build b/scripts/Makefile.build
    index 22adbf89cb31..41f3602fc8de 100644
    --- a/scripts/Makefile.build
    +++ b/scripts/Makefile.build
    @@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
    # LLVM assembly
    # Generate .ll files from .c
    quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
    - cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
    + cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<

    $(obj)/%.ll: $(src)/%.c FORCE
    $(call if_changed_dep,cc_ll_c)
    --
    2.38.0.413.g74048e4d9e-goog
    \
     
     \ /
      Last update: 2022-10-14 18:54    [W:4.869 / U:0.592 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site