[llvm-dev] Dealing with information loss for widened integer operations at ISel time

[Friedman, Eli via llvm-dev]

On 12/14/2018 3:43 AM, Alex Bradbury wrote:
> On Thu, 13 Dec 2018 at 21:41, Friedman, Eli <efriedma at codeaurora.org> wrote:
>> On 12/13/2018 6:25 AM, Alex Bradbury wrote:
>>> There's also likely to be cases where you want to calculate the demanded bits
>>> in order to determine if e.g. a W-suffixed instruction can be selected for
>>> `(somoeop (zexti32 GPR:$rs1), (zexti32 GPR:$rs2))`. This is easy to match if
>>> the SelectionDAG contains an explicit `sext_inreg` of the result. But if not,
>>> you'd need to check whether the upper 32 bits are actually demanded or not.
>> Could you describe more specifically where this matters?  I would guess
>> the W-suffixed instructions aren't actually any cheaper, in general,
>> than non-W-suffixed instructions, so there's no point to using W forms
>> if you don't need the sign extension. Unless MULW has lower latency than
>> MUL on some CPU?
> It only matters when it allows you to avoid an unnecessary sext/zext
> of the input operands. Consider the following input:
>
> define i32 @aext_divuw_aext_aext(i32 %a, i32 %b) nounwind {
>    %1 = udiv i32 %a, %b
>    ret i32 %1
> }
>
> This pattern won't match:
> def : Pat<(sext_inreg (udiv (zexti32 GPR:$rs1), (zexti32 GPR:$rs2)), i32),
>            (DIVUW GPR:$rs1, GPR:$rs2)>;
>
> This pattern would match but is incorrect in the general case (e.g. if
> rs1 is 0xffffffff and rs2 is 0x1, the result will be sign-extended).:
> def : Pat<(udiv (zexti32 GPR:$rs1), (zexti32 GPR:$rs2)),
>            (DIVUW GPR:$rs1, GPR:$rs2)>;
>
> So this function would generate:
> slli a1, a1, 32
> srli a1, a1, 32
> slli a0, a0, 32
> srli a0, a0, 32
> divu a0, a0, a1
> ret
>
> Rather than a simple divuw. Obviously we can argue whether such cases
> are likely to occur in real-world code (certainly this specific case
> of aext function args/returns isn't going to happen for
> clang-generated code), but it makes me uncomfortable that we can't
> easily make the best codegen decision for such a straight-forward
> input.

Okay, that makes sense.

I think you only end up with an explicit "zext" like this for unsigned 
division and unsigned right shift with a variable amount?  And for 
signed division, I guess you end up with an analogous problem with sext 
(since -2^31/-1 needs to produce 2^31, not -2^31).

For add/mul/etc. if the high result bits aren't demanded, the high 
operand bits also aren't demanded, so the extensions should be 
eliminated.  For unsigned right shift with a zero-extended operand and a 
constant non-zero shift amount, sign-extension is a no-op, so the 
obvious pattern just works. And for signed right shift, the result is 
always sign-extended, so the obvious pattern also just works.

For the nodes where it does matter, you could define RISCVISD::DIVUW 
etc. if necessary, I guess... and generate it using either custom 
lowering or SimplifyDemandedBitsForTargetNode.  I can't think of any 
other approach.

>>> Any thoughts on how to best handle these cases? For the shifts I could
>>> obviously introduce a target-specific SelectionDAG node, but then I'd lose the
>>> benefit of most of the target-independent DAG combines. A target DAG combine
>>> could custom lower the shift amount. e.g. converting (shl val, shamt) to (shl
>>> val, (and shamt, 31)) before it gets widened to i64. But this runs the risk
>>> of introducing an unnecessary AND operation if it turns out that the
>>> SelectionDAG node is transformed by the time it reaches instruction selection.
>>> So for this particular issue with shifts, introducing a target-specific WasI32
>>> node and converting to (shl val, (WasI32 shamt)) in a target DAG combine looks
>>> plausible.
>> You can represent "WasI32" using AssertZext i5.  That seems like a
>> reasonable approach.
> I hadn't considered AssertZext, thanks for the suggestion. Don't we
> really want AssertAnyExt i5 though (which doesn't currently exist)? We
> can't guarantee that the bits other than lower 5 are zero. I'd be
> willing to believe that there's no way this infidelity could cause
> issues for this particular transformation and the current dag
> combiner, but it still seems shaky.

The property you want is that the high bits of the shift amount are 
zero, or the result of the shift is undefined.  That matches "(shl x, 
(AssertZext i5 shamt))", as far as I know... maybe we could clarify the 
documentation?

Another alternative is to introduce RISCVISD::SHLW which is the same as 
ISD::SHL except it masks the bottom bits.  You potentially lose (or have 
to duplicate) some late DAGCombines, but we don't really have any 
interesting combines for shifts with a variable shift amount anyway.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project