[Bug target/93934] Unnecessary fld of uninitialized float stack variable results in ub of valid C++ code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93934

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-02-26
     Ever confirmed|0                           |1

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #5)
> Ah, indeed. fld won't raise FE_INVALID for 80-bit long double, but here
> 'result' is stored on the stack in 64-bit format.
> 
> So: fcmov and 80-bit fldt don't trap, 32-bit flds and 64-bit fldl do.
> 
> Somehow RTL if-conversion would have to check "-fsignaling-nans is requested
> and the target may raise FE_INVALID on loads" among other reason to reject a
> speculative load.
> 
> I am afraid though that several other optimizations do not anticipate that
> x87 fp loads can raise exceptions on SNaNs either, making -fsignaling-nans
> difficult to implement in full.

I suppose a fldt followed by "truncation" to 32/64 bit would then trap at
the truncation step?

But yes, this will be difficult to handle.  We'd have to mark all loads
from not must-initialized memory as possibly trapping and thus not eligible
for if-conversion.  And this applies to possibly uninitialized registers
as well which might be spilled or allocated to the stack.

So confirmed at least.  It's also more a middle-end issue affecting a
subset of targets only.