On 12/21/2024 03:40 PM, Xi Ruoyao wrote:
On Fri, 2024-12-20 at 15:34 -0700, Nathan Chancellor wrote:
Now, the thing is, these ASSERT()s are checking for divide-by-zero, I
suspect clang figured that out and invokes UB on us and just stops
code-gen.
Yeah, I think your analysis is spot on, as this was introduced by a
change in clang from a few months ago according to my bisect:
https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448
Since the ASSERT does not do anything to prevent the divide by zero (it
just flags it with WARN_ON) and the rest of the code doesn't either, I
assume that the codegen stops as soon as it encounters the unreachable
that change created from the path where divide by zero would occur via
dc_fixpt_recip() ->
dc_fixpt_from_fraction() ->
complete_integer_division_u64() ->
div64_u64_rem()
Shouldn't callers of division functions harden them against dividing by
zero?
Yes I think it'd be the correct solution.
Thank you all. Do you mean like this?
--- >8 ---
diff --git a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
index 88d3f9d7dd55..848d8e67304a 100644
--- a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
@@ -79,11 +79,13 @@ struct fixed31_32 dc_fixpt_from_fraction(long long
numerator, long long denomina
unsigned long long arg2_value = arg2_negative ? -denominator :
denominator;
unsigned long long remainder;
+ unsigned long long res_value;
/* determine integer part */
- unsigned long long res_value = complete_integer_division_u64(
- arg1_value, arg2_value, &remainder);
+ ASSERT(arg2_value);
+
+ res_value = complete_integer_division_u64(arg1_value,
arg2_value, &remainder);
ASSERT(res_value <= LONG_MAX);
@@ -214,8 +216,6 @@ struct fixed31_32 dc_fixpt_recip(struct fixed31_32 arg)
* Good idea to use Newton's method
*/
- ASSERT(arg.value);
-
return dc_fixpt_from_fraction(
dc_fixpt_one.value,
arg.value);
With the above changes, there is no "falls through" objtool warning
compiled with both clang 19 and the latest mainline clang 20.
If you are OK with it, I will send a separate formal patch to handle
this issue after doing some more testing.
Thanks,
Tiezhu
