[PATCH] D150100: [KnownBits] Improve implementation of `KnownBits::abs`
Nikita Popov via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Mon May 15 08:17:07 PDT 2023
nikic added inline comments.
================
Comment at: llvm/lib/Support/KnownBits.cpp:423
+ KnownBits KnownNeg = computeForAddSub(
+ /*Add*/ false, /*NSW*/ false, Zero, *this);
+
----------------
goldstein.w.n wrote:
> nikic wrote:
> > goldstein.w.n wrote:
> > > goldstein.w.n wrote:
> > > > nikic wrote:
> > > > > goldstein.w.n wrote:
> > > > > > nikic wrote:
> > > > > > > goldstein.w.n wrote:
> > > > > > > > nikic wrote:
> > > > > > > > > You can pass IntMinIsPoison to the NSW argument.
> > > > > > > > > You can pass IntMinIsPoison to the NSW argument.
> > > > > > > >
> > > > > > > > Can we? The `0-Negative_X` is positive unless `Negative_X == INT_MIN` so if `IntMinIsPoison` we can't set `NSW`. Also would do slightly better and just do `KnownAbs.isNonNegative()` because it uses the `IntMinIsPoison` condition (and others) to try and set output sign.
> > > > > > > >
> > > > > > > > But either way, since `KnownAbs` already has logic for setting output sign and we combine our result from negation with `KnownAbs` its kind of a non-issue.
> > > > > > > I don't understand. The only value for which `0-X` wraps is INT_MIN, which is exactly what the IntMinIsPoison flag controls. The entire purpose of that flag is to allow an `nsw` assumption on the negation.
> > > > > > Oh I mistakenly though something like `sub nsw i8 0, 129` would be `poison` but guess not.
> > > > > > Either way though, we 'join' with the original `abs` value to get the signbit so `nsw` is unneeded no?
> > > > > But can we drop that join if NSW is passed? Just making this `if (isNegative(X)) return neg(X);` would make this logic a lot cleaner.
> > > > > But can we drop that join if NSW is passed? Just making this `if (isNegative(X)) return neg(X);` would make this logic a lot cleaner.
> > > >
> > > > I doesn't seem to cover all cases:
> > > >
> > > > ```
> > > > KnownBits Zero(getBitWidth());
> > > > Zero.setAllZero();
> > > >
> > > > KnownBits KnownNeg = computeForAddSub(
> > > > /*Add*/ false, KnownAbs.isNegative(), Zero, *this);
> > > >
> > > > // Preserve signbit from `KnownAbs` as it has additional logic for figuring
> > > > // it out that we don't want to duplicate here.
> > > > KnownBits KnownAbsNoSign = KnownAbs;
> > > > KnownBits KnownNegNoSign = KnownNeg;
> > > > KnownAbsNoSign.One.clearSignBit();
> > > > KnownAbsNoSign.Zero.clearSignBit();
> > > > KnownNegNoSign.One.clearSignBit();
> > > > KnownNegNoSign.Zero.clearSignBit();
> > > >
> > > > assert(KnownAbsNoSign.One.isSubsetOf(KnownNegNoSign.One));
> > > > assert(KnownAbsNoSign.Zero.isSubsetOf(KnownNegNoSign.Zero));
> > > >
> > > > assert(KnownAbs.One.isSubsetOf(KnownNeg.One));
> > > > assert(KnownAbs.Zero.isSubsetOf(KnownNeg.Zero));
> > > >
> > > > KnownNeg.One.clearSignBit();
> > > > KnownNeg.Zero.clearSignBit();
> > > > KnownAbs.One |= KnownNeg.One;
> > > > KnownAbs.Zero |= KnownNeg.Zero;
> > > > ```
> > > >
> > > > On the exhaustive tests fails:
> > > >
> > > > ```
> > > > KnownBits.cpp:440: llvm::KnownBits llvm::KnownBits::abs(bool) const: Assertion `KnownAbs.Zero.isSubsetOf(KnownNeg.Zero)' failed.
> > > > ```
> > > > > But can we drop that join if NSW is passed? Just making this `if (isNegative(X)) return neg(X);` would make this logic a lot cleaner.
> > > >
> > > > I doesn't seem to cover all cases:
> > > >
> > > > ```
> > > > KnownBits Zero(getBitWidth());
> > > > Zero.setAllZero();
> > > >
> > > > KnownBits KnownNeg = computeForAddSub(
> > > > /*Add*/ false, KnownAbs.isNegative(), Zero, *this);
> > >
> > > This should be `isNonNegative()` but still fails the assertion.
> > > >
> > > > // Preserve signbit from `KnownAbs` as it has additional logic for figuring
> > > > // it out that we don't want to duplicate here.
> > > > KnownBits KnownAbsNoSign = KnownAbs;
> > > > KnownBits KnownNegNoSign = KnownNeg;
> > > > KnownAbsNoSign.One.clearSignBit();
> > > > KnownAbsNoSign.Zero.clearSignBit();
> > > > KnownNegNoSign.One.clearSignBit();
> > > > KnownNegNoSign.Zero.clearSignBit();
> > > >
> > > > assert(KnownAbsNoSign.One.isSubsetOf(KnownNegNoSign.One));
> > > > assert(KnownAbsNoSign.Zero.isSubsetOf(KnownNegNoSign.Zero));
> > > >
> > > > assert(KnownAbs.One.isSubsetOf(KnownNeg.One));
> > > > assert(KnownAbs.Zero.isSubsetOf(KnownNeg.Zero));
> > > >
> > > > KnownNeg.One.clearSignBit();
> > > > KnownNeg.Zero.clearSignBit();
> > > > KnownAbs.One |= KnownNeg.One;
> > > > KnownAbs.Zero |= KnownNeg.Zero;
> > > > ```
> > > >
> > > > On the exhaustive tests fails:
> > > >
> > > > ```
> > > > KnownBits.cpp:440: llvm::KnownBits llvm::KnownBits::abs(bool) const: Assertion `KnownAbs.Zero.isSubsetOf(KnownNeg.Zero)' failed.
> > > > ```
> > >
> > >
> > The argument to the NSW flag should be IntMinIsPoison, not KnownAbs.isNegative(). Effectively you're assuming that the flag is always set, so I would expect that to cause exhaustive test failures.
> Earlier in the code is:
> ```
> if (IntMinIsPoison || (!One.isZero() && !One.isMinSignedValue())) {
> KnownAbs.One.clearSignBit();
> KnownAbs.Zero.setSignBit();
> }
> ```
>
> So if `KnownAbs.isNonNegative()` is a superset of `IntMinPoison`.
Okay, I rewrote our test infrastructure to make optimality failures easier to analyze. For a simple implementation using
```
if (isNegative())
return computeForAddSub(/*Add*/ false, /*NSW*/ IntMinIsPoison,
KnownBits::makeConstant(APInt(getBitWidth(), 0)),
*this);
```
I get these failures:
```
/home/npopov/repos/llvm-project/llvm/unittests/Support/KnownBitsTest.cpp:86: Failure
Value of: isOptimal(Exact, Computed, Known)
Actual: false (Inputs = 1?00, Computed = 0?00, Exact = 0100)
Expected: true
/home/npopov/repos/llvm-project/llvm/unittests/Support/KnownBitsTest.cpp:86: Failure
Value of: isOptimal(Exact, Computed, Known)
Actual: false (Inputs = 10??, Computed = 0???, Exact = 01??)
Expected: true
/home/npopov/repos/llvm-project/llvm/unittests/Support/KnownBitsTest.cpp:86: Failure
Value of: isOptimal(Exact, Computed, Known)
Actual: false (Inputs = 10?0, Computed = 0??0, Exact = 0110)
Expected: true
/home/npopov/repos/llvm-project/llvm/unittests/Support/KnownBitsTest.cpp:86: Failure
Value of: isOptimal(Exact, Computed, Known)
Actual: false (Inputs = 100?, Computed = 0???, Exact = 0111)
Expected: true
```
Basically, the cases where it is non-optimal are cases where we don't make use of the fact that at least one of the unknown bits must be non-zero, otherwise the input would be IntMin. Those are differences in the non-sign bits though. And the optimality failures (for known negative numbers) that I get for your implementation with the KnownAbs fallback are exactly the same.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D150100/new/
https://reviews.llvm.org/D150100
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