[PATCH] D140851: [Patch 3/4]: Add cases for assume (X & Y != {0|Y})

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:701
+    if (!isValidAssumeForContext(I, Q.CxtI, Q.DT))
+      continue;
+
----------------
I believe the placement of this call was intentional, because it is more expansive than the icmp operand checks. That said, I don't see a compile-time impact on CTMark, though that is not exactly an assume-heavy workload.

We did address various inefficiencies in isValidAssumeForContext() over time though, so I would be fine with trying this change. It should be split out into a separate NFC patch though.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:939
+        // 1. is_pow2(b) && b == a
+        //      v.zeros[log2(b)] = 1
+        // 2. is_pow2(b) && 0 == a
----------------
It looks like we are missing this canonicalization: https://alive2.llvm.org/ce/z/MtveLU With that done, this becomes `v & b == 0` and is covered by existing handling.


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Comment at: llvm/lib/Analysis/ValueTracking.cpp:943
+        KnownBits BKnown =
+            computeKnownBits(B, Depth + 1, QueryNoAC).anyextOrTrunc(BitWidth);
+        if (isKnownToBeAPowerOfTwo(B, false, Depth + 1, QueryNoAC)) {
----------------
Move this computeKnownBits() call into the isKnownToBeAPowerOfTwo() branch.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:950
+            KnownBits AKnown = computeKnownBits(A, Depth + 1, QueryNoAC)
+                                   .anyextOrTrunc(BitWidth);
+            if (AKnown.isZero()) {
----------------
This should just check whether A is zero, no need to compute known bits.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D140851/new/

https://reviews.llvm.org/D140851