[llvm] bbeb084 - Revert "[GlobalISel] GISelKnownBits::computeKnownBitsImpl - Replace TargetOpcode::G_MUL handling with the common KnownBits::computeForMul implementation"

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2020-11-04T09:54:04-08:00
New Revision: bbeb08497ce5816bcd92989e21eea632993f8cea

URL: https://github.com/llvm/llvm-project/commit/bbeb08497ce5816bcd92989e21eea632993f8cea
DIFF: https://github.com/llvm/llvm-project/commit/bbeb08497ce5816bcd92989e21eea632993f8cea.diff

LOG: Revert "[GlobalISel] GISelKnownBits::computeKnownBitsImpl - Replace TargetOpcode::G_MUL handling with the common KnownBits::computeForMul implementation"

This reverts commit 0b8711e1af97d6c82dc9d25c12c5a06af060cc56 which broke GlobalISelTests AArch64GISelMITest.TestKnownBits

Added: 
    

Modified: 
    llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp b/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
index 862764f30522..81a89a6eb0b7 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
@@ -287,7 +287,20 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
                          Depth + 1);
     computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
                          Depth + 1);
-    Known = KnownBits::computeForMul(Known, Known2);
+    // If low bits are zero in either operand, output low known-0 bits.
+    // Also compute a conservative estimate for high known-0 bits.
+    // More trickiness is possible, but this is sufficient for the
+    // interesting case of alignment computation.
+    unsigned TrailZ =
+        Known.countMinTrailingZeros() + Known2.countMinTrailingZeros();
+    unsigned LeadZ =
+        std::max(Known.countMinLeadingZeros() + Known2.countMinLeadingZeros(),
+                 BitWidth) -
+        BitWidth;
+
+    Known.resetAll();
+    Known.Zero.setLowBits(std::min(TrailZ, BitWidth));
+    Known.Zero.setHighBits(std::min(LeadZ, BitWidth));
     break;
   }
   case TargetOpcode::G_SELECT: {