[llvm-dev] funnel shift, select, and poison

Sanjay Patel via llvm-dev
Mon, 25 Feb 2019 22:30:53 -0000

Don't we need to distinguish funnel shift from the more specific rotate?
I'm not seeing how rotate (a single input op shifted by some amount) gets
into trouble like funnel shift (two variables concatenated and shifted by
some amount).
Eg, if in pseudo IR we have:
%funnel_shift = fshl %x, %y, %sh ; this is problematic because either x or
y can be poison, but we may not touch the poison when sh==0
%rotate = fshl %x, %x, %sh ; if x is poison, the op is unquestionably
producing poison; there's no sh==0 loophole here


On Mon, Feb 25, 2019 at 1:12 PM Nuno Lopes <nunoplopes at sapo.pt> wrote:

> Thanks Sanjay!
>
> I did a quick study of these funnel shifts:
> The generic lowering to SDAG is correct for the optimization below. It
> actually stops poison if shift amount is zero:
>     SDValue ShAmt = DAG.getNode(ISD::UREM, sdl, VT, Z, BitWidthC);
> (...)
>     SDValue IsZeroShift = DAG.getSetCC(sdl, CCVT, ShAmt, Zero, ISD::SETEQ);
>     setValue(&I, DAG.getSelect(sdl, VT, IsZeroShift, IsFSHL ? X : Y,
Or));
>
> This is assuming select in SDAG stops poison in the same way we've
> proposed
> for the IR.
>
> However, the lowering has 2 optimizations. It can lower funnel shifts to:
> 1) A special funnel shift instruction if the backend supports it
> 2) Rotate
>
> At least lowering to rotate would be incorrect if rotate didn't stop
> poison
> as well when shift amount == 0. Most likely rotate doesn't block poison
> though. So this doesn't seem correct.
>
> Blocking poison, while tempting, is usually not a good idea because it
> blocks many optimizations. It becomes hard to remove instructions that
> block
> poison. Exactly as you see here: if select blocks poison (and we claim it
> does), then it cannot be removed.
>
> I have 2 separate proposals:
> 1) Keep select blocking poison, and remove the transformation below
> because
> it doesn't play well with 1) lowering to rotate, and 2) because it
blocks
> optimizations like converting funnel shifts to plain shifts
> 2) Introduce a flag in select, like we have nsw/nuw today that changes the
> semantics regarding poison. Essentially a select that doesn't stop
poison.
> This can be safely emitted by most frontends of the languages we support
> today, but wouldn't be used by compiler-introduced selects. The
> optimization
> below would only kick in when this flag is present. Of course then we can
> have an analysis that inserts these flags like we have for nsw.
>
> I like 2), but 1) is simpler. I don't know if 2) is worth it, but is
> appealing :)
>
> Nuno
>
>
> -----Original Message-----
> From: Sanjay Patel via llvm-dev
> Sent: Monday, February 25, 2019 4:29 PM
> Subject: [llvm-dev] funnel shift, select, and poison
>
>
> There's a question about the behavior of funnel shift [1] + select and
> poison here that reminds me of previous discussions about select and
> poison
> [2]:
> https://github.com/AliveToolkit/alive2/pull/32#discussion_r257528880
>
> Example:
> define i8 @fshl_zero_shift_guard(i8 %x, i8 %y, i8 %sh) {
> %c = icmp eq i8 %sh, 0
> %f = fshl i8 %x, i8 %y, i8 %sh
> %s = select i1 %c, i8 %x, i8 %f ; shift amount is 0 returns x (same as
> fshl)
> ret i8 %s
> }
> =>
> define i8 @fshl_zero_shift_guard(i8 %x, i8 %y, i8 %sh) {
> %f = fshl i8 %x, i8 %y, i8 %sh
> ret i8 %f
> }
> Transformation doesn't verify!
> ERROR: Target is more poisonous than source
>
>
>
----------------------------------------------------------------------------
>
> The problem is that if %y is poison and we assume that funnel shift uses
> all
> of its operands unconditionally, the reduced code sees poison while the
> original code is protected by the "conditional poison"
(terminology?)
> property of a select op and is safe.
>
> If we treat funnel shift more like a select based on its operation (when
> the
> shift amount is 0, we know that the output is exactly 1 of the inputs),
> then
> the transform should be allowed?
>
> This transform was implemented in instcombine [3] with the motivation of
> reducing UB-safe rotate code in C to the LLVM intrinsic [4]. So a
> potential
> sidestep of the problem would be to limit that transform to a rotate
> pattern
> (single value is being shifted) rather than the more general funnel
> pattern
> (two values are being shifted).
>
> [1] https://llvm.org/docs/LangRef.html#llvm-fshl-intrinsic
> [2] http://llvm.1065342.n5.nabble.com/poison-and-select-td72262.html
> [3] https://reviews.llvm.org/D54552
> [4] https://bugs.llvm.org/show_bug.cgi?id=34924
>
>
>
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