DenisTarasyuk commented on issue #46708:
URL: https://github.com/apache/arrow/issues/46708#issuecomment-2955947276
Additional information.
I think this issue happens due to undefined behavior in castDECIMAL_utf8.
This code does not set out_high, out_low.
```
int32_t status =
gdv_fn_dec_from_string(context, in, in_length, &precision_from_str,
&scale_from_str,
&dec_high_from_str, &dec_low_from_str);
if (status != 0) {
return;
}
```
And LLVM decides to optimize out some parts of code due to this behavior.
In linked PR there is test that reproduces issue.
Here I link some IR code.
[ir_bug_raw.txt](https://github.com/user-attachments/files/20656070/ir_bug_raw.txt)
is IR code with bug reproduced. This IR code '_raw' was captured before any
optimisations applied by LLVM.
[ir_fixed_raw.txt](https://github.com/user-attachments/files/20656068/ir_fixed_raw.txt)
is non optimized IR with fix for out_high, out_low set to 0.
Then I used LLVM opt command like this:
`/llvm/bin/opt -passes='default<O1>' -debug-pass-manager -S ir_fixed_raw.txt
-o ir_fixed_O1.txt`
This produces optimised IR code.
Here are original and fixed IR with O1 optimisation level. They both work
fine.
[ir_bug_O1.txt](https://github.com/user-attachments/files/20656069/ir_bug_O1.txt)
[ir_fixed_O1.txt](https://github.com/user-attachments/files/20656071/ir_fixed_O1.txt)
Here are the same but with O3 level. And here original IR has issue:
[ir_bug_O3.txt](https://github.com/user-attachments/files/20656073/ir_bug_O3.txt)
[ir_fixed_O3.txt](https://github.com/user-attachments/files/20656072/ir_fixed_O3.txt)
The issue is here:
Original:
```
%61 = call i32 @gdv_fn_dec_from_string(i64 noundef %context_ptr, ptr noundef
nonnull @0, i32 noundef 3, ptr noundef nonnull %11, ptr noundef nonnull %12,
ptr noundef nonnull %9, ptr noundef nonnull %10)
%62 = icmp eq i32 %61, 0
call void @llvm.assume(i1 %62)
call void @llvm.lifetime.start.p0(i64 24, ptr nonnull %13) #7
```
Fixed:
```
%61 = call i32 @gdv_fn_dec_from_string(i64 noundef %context_ptr, ptr noundef
nonnull @0, i32 noundef 3, ptr noundef nonnull %11, ptr noundef nonnull %12,
ptr noundef nonnull %9, ptr noundef nonnull %10)
%62 = icmp eq i32 %61, 0
br i1 %62, label %63, label %castDECIMAL_utf8.exit
```
If I understand correctly in original IR LLVM just assumed that
gdv_fn_dec_from_string should always return 0. So code that should have been
skipped by return is now executed. This and uninitialised out_high, out_low
cause SIGSEGV later.
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