Re: [PATCH 0/6] PowerPC Dense Math prelimary support (-mcpu=future)

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, Jan 31, 2023 at 10:31:03PM -0500, Michael Meissner wrote:
> Ok, I tracked down the source of the bug.  The CCP pass is depending on the
> precision field.  Unfortunately in tree-core.h, the precision is a 10 integer
> bit field, so 1,024 will become 0.
> 
> Having a 0 precision meant that the hwint function for sign extending a value
> would generate:
> 
> 	(HOST_WIDE_INT)(((unsigned HOST_WIDE_INT)value << 64) >> 64)
> 
> which is undefined behavior in C and C++.  On the x86_64 doing the shift left
> and then right gives you the initial value (which was -1), while on the PowerPC
> it always gives you 0.  The CCP code was assuming if it wasn't -1, that it was
> an integer, but the TDO type is opaque, not integer.

Variable 64-bit shifts on x86 mask the shift amount to 6 bits, while on
PowerPC it is masked to 7 bits.  It sounds like that is what you hit,
with some -O0 build perhaps.  But either way UB is UB, the program has
no meaning, any output is correct, no output is correct as well :-)
Nasal demons and all that.

bootstrap-ubsan should have found this?


Segher