Cryptic bug for DMD v2.087.x

[Nicholas Wilson]

On Monday, 12 August 2019 at 01:45:24 UTC, Peter Jacobs wrote:
> We have a flow simulation code 
> (http://cfcfd.mechmining.uq.edu.au/eilmer/) that fails to build 
> optimized with DMD v2.087.0 or v2.087.1.  The cryptic message 
> at the end of the attempt to build is:
>   tym = x14
>   makefile:414: recipe for target 'e4shared' failed
>   make: *** [e4shared] Illegal instruction (core dumped)
> and the compiler flags are:
>   dmd -w -O -release -inline -boundscheck=off -dip1008 ...
>
> The build is successful if we omit the -O flag.
>
> In contrast, we have successful optimized builds for DMD 
> 2.086.1 and LDC 1.17.0-beta1.
>
> I can provide more details but I was thinking that the "tym = 
> x14" might already mean something to someone familiar with the 
> insides of DMD.
>
> Best regards,
> Peter J.

That looks like 
https://github.com/dlang/dmd/blob/eda232a9f72b482c92cefb27a4321afc0aedbb5c/src/dmd/backend/cgxmm.d#L816-L817 or https://github.com/dlang/dmd/blob/eda232a9f72b482c92cefb27a4321afc0aedbb5c/src/dmd/backend/cgxmm.d#L716-L717 (or maybe https://github.com/dlang/dmd/blob/8623bbbebe35b9a320ba20e521a6c0ee51a780ac/src/dmd/backend/evalu8.d#L905 )

Can you run it in a debugger and provide a stack trace to confirm?

tym = 0x14 corresponds to real (== 80 bit fp), which is best 
avoided if possible as it is very slow.

Also, silly question, why would you use DMD, with optimisations, 
for cfd? surely your productivity is runtime bottlenecked?