Re: [PATCH] warn when zero-extending a negation

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On Thu, Dec 17, 2020 at 12:51:52AM +0100, Luc Van Oostenryck wrote:
> But replacing a trunc + zext by the corresponding masking, very
> little, if anything is done for such 'mixed-width' expressions.
> So, I'm even a bit surprised by the not.8 but well ... 

This bothered me a bit and kept me awake, so I had to check.

I think that the situation is caused by some premature optimization
for the ~ operator in expression.c:cast_to(). It saves the allocation
and initialization of one expression but makes things more complicated
at linearization and simplification. If this is disabled, then the IR
simplification returns what I was expecting:
		zext.32     %r2 <- (8) %arg1
		shl.32      %r5 <- $1, %arg2
		not.32      %r6 <- %r5
		and.32      %r9 <- %r6, $255
		and.32      %r10 <- %r2, %r9
		trunc.8     %r11 <- (32) %r10
		ret.8       %r11

and some reassociation patches (coming soon) will simplify away
the masking with $255 and the trunc.8

-- Luc

[Index of Archives]     [Newbies FAQ]     [LKML]     [IETF Annouce]     [DCCP]     [Netdev]     [Networking]     [Security]     [Bugtraq]     [Yosemite]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux SCSI]     [Trinity Fuzzer Tool]

  Powered by Linux