[PATCH] D109883: [Analysis] Add support for vscale in computeKnownBitsFromOperator

[David Sherwood via Phabricator via llvm-commits]

david-arm added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1708
+
+        unsigned FirstSetBit = findFirstSet(VScaleRange.first);
+        if (FirstSetBit <= BitWidth)
----------------
foad wrote:
> Can min ever be zero here?
Yeah min can be zero. I think see the problem now because FirstSetBit will be set to UINT_MAX.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1710
+        if (FirstSetBit <= BitWidth)
+          Known.Zero.setLowBits(FirstSetBit);
+
----------------
foad wrote:
> This assumes that vscale is a multiple of min, which is not documented in the langref.
Yes you're absolutely right. Of course if min=4, then a value of 5 would still set bit 0 too. I think that means I can only use min to check if it's equal to max.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D109883/new/

https://reviews.llvm.org/D109883