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* [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll_c
@ 2022-10-07 20:32 Nick Desaulniers
  2022-10-13 19:56 ` Nathan Chancellor
  0 siblings, 1 reply; 7+ messages in thread
From: Nick Desaulniers @ 2022-10-07 20:32 UTC (permalink / raw)
  To: Masahiro Yamada
  Cc: Michal Marek, Nathan Chancellor, Tom Rix, linux-kbuild,
	linux-kernel, llvm, Nick Desaulniers

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.

Implicit Default (-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]

Explicit -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

Clang defaults to -fdiscard-value-names to save memory when generating
LLVM IR. For debugging purposes, the improvement in readability at a
cost of more verbose IR is a cost we're happy to pay.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 scripts/Makefile.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 27be77c0d6d8..d0e4f476dfee 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)

base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
-- 
2.38.0.rc2.412.g84df46c1b4-goog


^ permalink raw reply related	[flat|nested] 7+ messages in thread

end of thread, other threads:[~2022-10-14 16:53 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2022-10-07 20:32 [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll_c Nick Desaulniers
2022-10-13 19:56 ` Nathan Chancellor
2022-10-13 20:41   ` Fangrui Song
2022-10-13 20:53     ` Nick Desaulniers
2022-10-14 13:30       ` Masahiro Yamada
2022-10-14 16:47         ` [PATCH v2] " Nick Desaulniers
2022-10-14 16:53           ` [PATCH v3] " Nick Desaulniers

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