wasmtime / PR #6196 egraphs: Add `bmask` bit pattern opti... · git-wasmtime

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Topic: wasmtime / PR #6196 egraphs: Add `bmask` bit pattern opti...

Wasmtime GitHub notifications bot (Apr 11 2023 at 10:54):

afonso360 opened PR #6196 from afonso360:long-bmask to bytecodealliance:main:

:wave: Hey,

This is something I wrote in #5888 and then @elliottt pointed out that It's just bmask. I checked if we had a rule for this in the mid end and we don't.

This is a really long long rule, so I'm not sure how often it's going to fire. But I also got that lowering from here so it is out there in the real world.

This is based on top of #6140 so that we don't have to rebase that again.

Wasmtime GitHub notifications bot (Apr 11 2023 at 10:54):

afonso360 requested fitzgen for a review on PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 10:54):

afonso360 requested wasmtime-compiler-reviewers for a review on PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 10:55):

afonso360 edited PR #6196:

:wave: Hey,

This is something I wrote in #5888 and then @elliottt pointed out that It's just bmask. I checked if we had a rule for this in the mid end and we don't.

This is a really long long rule, and I'm not sure how often it's going to fire. But I also got that lowering from here so it is out there in the real world.

This is based on top of #6140 so that we don't have to rebase that again.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:01):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:01):

afonso360 edited PR #6196:

:wave: Hey,

This is something I wrote in #5888 and then @elliottt pointed out that It's just bmask. I checked if we had a rule for this in the mid end and we don't.

This is a really long long rule, and I'm not sure how often it's going to fire. But I also got that lowering from here so it is out there in the real world.

This is based on top of #6140 so that we don't have to rebase that again.

I'm going to let the fuzzer run on this for a while.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:01):

afonso360 edited PR #6196:

:wave: Hey,

This is something I wrote in #5888 and then @elliottt pointed out that It's just bmask. I checked if we had a rule for this in the mid end and we don't.

This is a really long rule, and I'm not sure how often it's going to fire. But I also got that lowering from here so it is out there in the real world.

This is based on top of #6140 so that we don't have to rebase that again.

I'm going to let the fuzzer run on this for a while.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:16):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:18):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 11:24):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:00):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:00):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:00):

fitzgen created PR review comment:

Is this actually correct? I couldn't convince myself, so I asked Z3 to find an x for which bmask(x) and !(((x | ((!x) + 1)) >> (ty_bits - 1)) - 1) were not equal (that is, I asked it to find a counterexample):

(declare-const x (_ BitVec 32))

(assert (! (= (ite (= x #x00000000)
                   #x00000000
                   #xffffffff)
              ;; !(((x | ((!x) + 1)) >> 31) - 1)
              (bvnot (bvsub (bvlshr (bvor x
                                          (bvadd (bvnot x)
                                                 #x00000001))
                                    #x0000001f)
                            #x00000001)))))

(check-sat)
(get-model)

It reports sat, meaning that it found a counterexample, and it says the counterexample is zero:

sat
(
  (define-fun x () (_ BitVec 32)
    #x00000000)
)

So unless I'm missing something and/or translated the expression incorrectly, I think this optimization is incorrect?

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:01):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:01):

fitzgen created PR review comment:

Then again, there is an explicit test for 0 in the run tests, so I must have done something wrong...

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:14):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:14):

cfallin created PR review comment:

The SMT counterexample is confusing me too:

x1 := (bvnot x) with x := 0 -> 0xffffffff

x2 := (bvadd x1 1) -> 0

x3 := (bvor x2 x) -> (bvor 0 0) -> 0

x4 := (bvlshr x3 31) -> 0

x5 := (bvsub x4 1) -> 0xffffffff

x6 := (bvnot x5) -> 0

and on the LHS for the if-then-else we have (ite (= 0 0) 0 0xffffffff) -> 0. Seems like the optimization did what it was supposed to? I too am confused why we get SAT here.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:19):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:19):

jameysharp created PR review comment:

That Z3 expression looks like a correct translation of that pattern to my eyes, so I'm confused too.

I think the rule seems correct:

!input + 1 is equivalent to -input in two's-complement.

0 is the only value which, when negated, leaves the sign bit clear. (INT_MIN leaves the sign bit set; all other cases flip the sign bit.)

Shifting the sign bit down after x | -x then gives us 0 if the input was 0, and 1 otherwise.

Subtracting 1 means we get -1 if the input was 0, and 0 otherwise.

Inverting the result should then give us the right answer.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:23):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:23):

cfallin created PR review comment:

And I think this is a reasonable informal proof, by cases:

Prove: ;; bmask(input) = !(((input | ((!input) + 1)) >> 63) - 1) (for 64-bit input)

Take the three cases

input == 0: we have the above evaluation; the RHS produces 0 and bmask(0) == 0.

input < 0: input has MSB = 1, thus input | ... has MSB = 1. Right-shift by 63, we have 1; 1 - 1 = 0; outer bnot gives us all-ones.

input > 0: input has MSB = 0, thus !input has MSB = 1. Furthermore !input != 0xffff...ffff (else we would have had input == 0). So !input + 1 also has MSB = 1 (because it will not wrap around, because of previous statement). So input | ((!input) + 1) has MSB = 1. Right-shift by 63, we have 1; 1 - 1 = 0; outer bnot gives us all-ones.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:24):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:24):

cfallin created PR review comment:

(@afonso360 a comment to this effect in the source would be very useful for the future!)

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:27):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:27):

cfallin created PR review comment:

Ah, and after posting I see @jameysharp 's comment too. Both of us arrived at the same reasoning more or less so I'm fairly confident here :-)

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:46):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:46):

jameysharp created PR review comment:

We can extract out more broadly-applicable rules that should compose nicely into this one.

Rewrite both of these patterns to (ineg ty input):

(iadd ty (bnot ty input) (iconst ty (u64_from_imm64 1)))

(bnot ty (isub ty input (iconst ty (u64_from_imm64 1))))

In a 64-bit word, -(x >> 63), where right-shift is unsigned, should I think be equivalent to x >> 63 with a signed right-shift. Or in general, rewrite negation of unsigned shifts by N-1 bits in N-bit words.

Then this rule only needs to match (x | -x) >> (N-1), with a signed right-shift.

That should make all these rules more broadly applicable as well as (somewhat) easier to understand.

x | -x leaves the trailing 0s alone and sets the remaining bits to 1. (I found the inverse of this pattern in "Hacker's Delight", on page 12.) I don't think there's a useful rule we can extract from that pattern alone.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:49):

elliottt submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 18:49):

elliottt created PR review comment:

The problem with the smtlib version was the use of !, which is for annotating expressions: switching to not fixes the problem:

(declare-const x (_ BitVec 32))

(assert (not (= (ite (= x #x00000000)
                   #x00000000
                   #xffffffff)
              ;; !(((x | ((!x) + 1)) >> 31) - 1)
              (bvnot (bvsub (bvlshr (bvor x
                                          (bvadd (bvnot x)
                                                 #x00000001))
                                    #x0000001f)
                            #x00000001)))))

(check-sat)
(get-model)

produces the following output:

% z3 -smt2 test.smt
unsat
(error "line 14 column 10: model is not available")

Wasmtime GitHub notifications bot (Apr 11 2023 at 19:08):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 19:08):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 19:08):

fitzgen created PR review comment:

Nitpick: instead of hard coding 63 lets make this bits(ty) - 1.

Also, lets add a comment with an informal proof / argument about why this is correct, as @cfallin mentioned.

Wasmtime GitHub notifications bot (Apr 11 2023 at 19:36):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Apr 11 2023 at 19:36):

jameysharp created PR review comment:

I think the correctness argument is much less important to write out if this is split into the four separate rules I suggested. They're individually much easier to verify.

Wasmtime GitHub notifications bot (Apr 13 2023 at 13:10):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 13 2023 at 14:09):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 13 2023 at 14:17):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp created PR review comment:

This comment is great, thank you!

I guess we should add the commutative version of this too, for (-x | x), to go with (x | -x).

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp created PR review comment:

I'd like a couple more cases, maybe instead of the cases for 2 or 3:

0xFFFF_FFFF_FFFF_FFFF

0x8000_0000_0000_0000

I'd also be fine with narrowing this to i32 or i16 so the interesting edge cases have smaller constants…

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp created PR review comment:

This rule would also work for (x + (-1)) and its commutative equivalent.

I considered suggesting that we should have a rule that transforms isub where the second operand is constant into iadd of the negation of that constant. Then any rules involving addition with a constant can fire for isub as well. For example, (!x) - (-1) would fire the rules above.

That's not ideal though since it replaces an instruction with another instruction of equal cost, and I think we're converging on the idea that every egraph rule should rewrite to an expression of lower total cost. So in this case we'd want six rules in total:

!x + 1 == 1 + !x == !x - (-1) == -x

!(x - 1) == !(x + (-1)) == !((-1) + x) == -x

Wasmtime GitHub notifications bot (Apr 13 2023 at 18:58):

jameysharp created PR review comment:

Elsewhere in this file these comments were written in the other order, with the pattern we're looking for first and the simpler equivalent afterward, and I think that makes a little more sense. So this example would instead be:
;; (!x) + 1 == ineg(x)
And similarly through the rest of the rules in this PR.

Wasmtime GitHub notifications bot (Apr 13 2023 at 19:51):

afonso360 updated PR #6196.

Wasmtime GitHub notifications bot (Apr 13 2023 at 19:52):

afonso360 submitted PR review.

Wasmtime GitHub notifications bot (Apr 14 2023 at 19:29):

jameysharp merged PR #6196.

Last updated: Dec 23 2024 at 12:05 UTC