How to track down a bad llvm optimization pass

[Joakim via digitalmars-d-ldc]

On Tuesday, 21 June 2016 at 19:41:59 UTC, Dan Olson wrote:
> On Tuesday, 21 June 2016 at 07:00:35 UTC, Joakim wrote:
>> On Tuesday, 21 June 2016 at 05:51:56 UTC, Joakim wrote:
>>> On Monday, 20 June 2016 at 20:15:52 UTC, Rainer Schuetze 
>>> wrote:
>>>>
>>>>
>>>> On 19.06.2016 08:58, Joakim wrote:
>>>>> [...]
>>>>
>>>> If it fails in a specific optimization pass, it has already 
>>>> passed a number of verification passes. When I posted some 
>>>> bug reports, the LLVM people (who have been very helpful) 
>>>> considered it an LLVM bug if an invalid IR passes the 
>>>> verifier. So even if some assumptions are broken, I would 
>>>> suggest to report to the LLVM bug tracker.
>>>
>>> OK, will do.  I think the fact that the exact same ldc 
>>> frontend linked with llvm 3.7.1 doesn't have this problem, 
>>> only when linked against llvm 3.8.0, indicates this most 
>>> likely isn't a problem on our end.
>>
>> Filed: https://llvm.org/bugs/show_bug.cgi?id=28224
>
> Hmmm.  I looked at failing code in std.conv.  I think it does a 
> negative shift, which is undefined in C.  It is the opSlice() 
> method.

I just tried the three-line reduced test case on linux/x64 with 
the official ldc 1.0.0 release linked against 3.8.0 and it 
asserts there too.  However, if I compile all of the std.conv 
unittests and run them all, they all pass.

It appears that you've found a clear off-by-one bug.  I didn't 
investigate our end because the tests were all passing otherwise- 
I now see that dmd doesn't assert even against the reduced test 
case- but I should have examined that code more closely.  It 
would have helped if llvm and/or the ddmd frontend had flagged 
that Phobos was shifting by a negative number, rather than 
passing it through silently for either correct or junk codegen.

I agree that the real fix for this bug should go in Phobos, but 
perhaps we can also improve the spots where this negative shift 
was most silently discarded so far, whether ddmd or llvm.