[prompt, win32] Read-after-write of variable of type `double` does not yield the correct result

[linev]

Typing in ROOT prompt one gets:

root [1] double z = 1234;
root [2] z = 1
(double) 1.00000
root [3] z
(double) 1234.0000

Last line is wrong - expected of course 1.0000.
Works ok on Linux and on Win64 (according to @bellenot)

[bellenot]

OK, so it works with bare cling:

C:\Users\bellenot\build\x86\llvm>Release\bin\cling

****************** CLING ******************
* Type C++ code and press enter to run it *
*             Type .q to exit             *
*******************************************
[cling]$ double z = 1234;
[cling]$ z = 1
(double) 1.0000000
[cling]$ z
(double) 1.0000000
[cling]$ .q

[bellenot]

@axel @jalopezg-r00t what ROOT adds on top of Cling that could cause this issue? (just for me to know where to look at...)
BTW, that works fine if in a macro (even unnamed)

[jalopezg-git]

Last line is wrong - expected of course 1.0000.

Which version and platform are you running ROOT on?
This is what I get with ROOT (Linux x86_64) built from the master branch:

root [0] double z = 1234;
root [1] z = 1
(double) 1.0000000
root [2] z
(double) 1.0000000

[bellenot]

Windows 32 bit (i.e. it Works on Windows 64 bit)

[jalopezg-git]

Can I have remote access to that ROOT build on a Windows host? Perhaps, you could also try this and report whether it gives the correct result:

root [0] gClingOpts->AllowRedefinition = 0;
root [1] double z = 1234;
root [2] z = 1
(double) 1.0000000
root [3] z
(double) ????

[bellenot]

It's on my local computer, but I can give you access to a VM (tomorrow)

C:\Users\bellenot>root -l
root [0] gClingOpts->AllowRedefinition = 0;
root [1] double z = 1234;
root [2] z = 1
(double) 1.0000000
root [3] z
(double) 1234.0000
root [4]

[jalopezg-git]

It's on my local computer, but I can give you access to a VM (tomorrow)

Yes, let's do that tomorrow! 👍

[jalopezg-git]

The issue only occurs in 32-bit Windows. As discussed, it is related to an optimization pass (-O1 is enabled for ROOT, but not for standalone cling). Both workarounds below yield the correct result; the exact pass triggering this is to be determined.

root [0] .O 0
root [1] double z = 1234;
root [2] z = 1
(double) 1.0000000
root [3] z
(double) 1.0000000

or

root [0] volatile double z = 1234;
// ...

As discussed with @Axel-Naumann, we'll probably defer this after the llvm upgrade.