[all-commits] [llvm/llvm-project] 625538: [KnownBits] Fix add() SelfAdd assertion for bitwid...

[Paweł Bylica via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 625538f44601b906a3db03b2b110f50915ca3ecd
      https://github.com/llvm/llvm-project/commit/625538f44601b906a3db03b2b110f50915ca3ecd
  Author: Paweł Bylica <pawel at hepcolgum.band>
  Date:   2026-06-11 (Thu, 11 Jun 2026)

  Changed paths:
    M llvm/lib/Support/KnownBits.cpp
    M llvm/unittests/Support/KnownBitsTest.cpp

  Log Message:
  -----------
  [KnownBits] Fix add() SelfAdd assertion for bitwidths >= 512 (#202769)

`KnownBits::add()` with `SelfAdd=true` lowers `X+X` to `shl(X, 1)` using
a fixed 8-bit shift amount:

```cpp
KnownBits Amt = KnownBits::makeConstant(APInt(8, 1));
return KnownBits::shl(LHS, Amt, NUW, NSW, /*ShAmtNonZero=*/true);
```

The comment there claims the shift-amount bitwidth is independent of the
source bitwidth, but that is not true: `shl()`'s `getMaxShiftAmount()`
extracts `Log2_32(BitWidth)` bits from the shift amount's max value when
`BitWidth` is a power of two:

```cpp
static unsigned getMaxShiftAmount(const APInt &MaxValue, unsigned BitWidth) {
  if (isPowerOf2_32(BitWidth))
    return MaxValue.extractBitsAsZExtValue(Log2_32(BitWidth), 0);
  ...
}
```

For source widths `>= 512`, `Log2_32(BitWidth) >= 9`, so extracting that
many bits from the 8-bit shift amount trips the assertion in
`APInt::extractBitsAsZExtValue`:

```
Assertion `bitPosition < BitWidth && (numBits + bitPosition) <= BitWidth && "Illegal bit extraction"' failed.
 #9  llvm::APInt::extractBitsAsZExtValue(unsigned int, unsigned int) const
#10  llvm::KnownBits::shl(...)
#11  llvm::KnownBits::add(...)
```

(256 is the boundary that still works: `Log2_32(256) == 8`, extracting
exactly 8 bits from the 8-bit amount.)

Fix: make the shift-amount bitwidth match the source bitwidth so the
extraction is always in bounds. The `SelfAdd` path was introduced in
#188078.

Found via fuzzing. Adds a `SelfAddWide` regression test covering widths
256/512/1024.



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