[PATCH] D142271: [ValueTracking] Add KnownBits patterns `xor(x, x - 1)` and `and(x, -x)` for knowing upper bits to be zero

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1093
+      if (match(I, m_c_BinOp(m_Value(X), m_Neg(m_Deferred(X))))) {
+        if (Known.countMaxTrailingZeros() <= Known2.countMaxTrailingZeros())
+          Known = Known.blsi();
----------------
Add a comment that as `-(-x) == x`, we can view this as a blsi operation on either operand, and pick whichever produces the best result.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1096
+        else
+          Known = Known2.blsi();
+      }
----------------
break here? Don't think we want to fall through.


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Comment at: llvm/lib/Analysis/ValueTracking.cpp:1134
+    if ((!Known.One.isZero() || !Known2.One.isZero()) &&
+        match(I, m_c_BinOp(m_Value(X), m_c_Add(m_Deferred(X), m_APInt(C)))) &&
+        C->isAllOnes()) {
----------------
`m_APInt(C)` -> `m_AllOnes()`.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1136
+        C->isAllOnes()) {
+      const KnownBits &XBits = I->getOperand(0) == X ? Known2 : Known;
+      Known = XBits.blsmsk();
----------------
Shouldn't this be `? Known : Known2`? We want the known bits of X, not of X-1.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D142271/new/

https://reviews.llvm.org/D142271