On 2022-10-13, Nathan Chancellor wrote:
On Fri, Oct 07, 2022 at 01:32:36PM -0700, Nick Desaulniers wrote:
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.
A LLVM_ENABLE_ASSEERTIONS=off build of Clang defaults to -fdiscard-value-names.
A LLVM_ENABLE_ASSEERTIONS=on build of Clang defaults to -fno-discard-value-names.
See clang/lib/Driver/ToolChains/Clang.cpp:5030
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Thanks for the patch!
Link: https://github.com/ClangBuiltLinux/linux/issues/1467
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
---
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
