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 -fdiscard-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@xxxxxxxxxx>
Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
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
