From aae068d048c3a8a1541c23b92f6eec69df14c776 Mon Sep 17 00:00:00 2001
From: Nandor Licker <n@ndor.email>
Date: Thu, 9 Nov 2023 09:40:38 -0800
Subject: [PATCH] Revert "[FIRRTL] Simplify muxes when a particular bit value
 selects the same value. (#6382)"

This reverts commit e49b521e5946e03f8db0b0f16523e0740a638094.
---
 lib/Dialect/FIRRTL/FIRRTLFolds.cpp        | 62 -----------------------
 test/Dialect/FIRRTL/canonicalization.mlir |  9 ----
 2 files changed, 71 deletions(-)

diff --git a/lib/Dialect/FIRRTL/FIRRTLFolds.cpp b/lib/Dialect/FIRRTL/FIRRTLFolds.cpp
index aec90c670531..dbc78f00bce8 100644
--- a/lib/Dialect/FIRRTL/FIRRTLFolds.cpp
+++ b/lib/Dialect/FIRRTL/FIRRTLFolds.cpp
@@ -1692,68 +1692,6 @@ LogicalResult MultibitMuxOp::canonicalize(MultibitMuxOp op,
     }
   }
 
-  // Eliminate unneeded bits in the index.  These arise from duplicate values in
-  // the mux.  This is done by slicing the mux into a mux tree.
-  // multibit_mux(index, {a, b, a, c}) -> multibit_mux(index[0],
-  // multibit_mux(index[1], a,a), multibit_mux(index[1]}, {b,c})) Search for
-  // identities in specific bit slices.   This is robust to unknown width
-  // indexes.
-  for (uint64_t bit = 0,
-                lastbit = op.getIndex().getType().getBitWidthOrSentinel();
-       bit < lastbit; ++bit) {
-    for (int curval = 0; curval <= 1; ++curval) {
-      // We don't collect values here as the normal case is we don't find a
-      // match, so we don't want to move data around and do allocations.
-      Value v;
-      uint64_t count = 0;
-      for (uint64_t i = 0, e = op.getInputs().size(); i < e; ++i) {
-        if (((i >> bit) & 1) != curval)
-          continue;
-        ++count;
-        if (!v)
-          v = op.getInputs()[i];
-        if (v != op.getInputs()[i]) {
-          v = {};
-          break;
-        }
-      }
-      if (!v || count == 1)
-        continue;
-      // Found match, collect varying side of the future mux
-      SmallVector<Value> nonSimple;
-      for (uint64_t i = 0, e = op.getInputs().size(); i < e; ++i) {
-        if (((i >> bit) & 1) != curval)
-          nonSimple.push_back(op.getInputs()[i]);
-      }
-      Value indBit = rewriter.createOrFold<BitsPrimOp>(op.getLoc(),
-                                                       op.getIndex(), bit, bit);
-      Value indBitRemLow;
-      if (bit)
-        indBitRemLow = rewriter.createOrFold<BitsPrimOp>(
-            op.getLoc(), op.getIndex(), bit - 1, 0);
-      else
-        indBitRemLow = rewriter.create<ConstantOp>(
-            op.getLoc(), IntType::get(op.getContext(), false, 0),
-            APInt(0U, 0UL));
-      Value indBitRemHigh;
-      if (bit == lastbit - 1)
-        indBitRemHigh = rewriter.create<ConstantOp>(
-            op.getLoc(), IntType::get(op.getContext(), false, 0),
-            APInt(0U, 0UL));
-      else
-        indBitRemHigh = rewriter.createOrFold<BitsPrimOp>(
-            op.getLoc(), op.getIndex(), lastbit - 1, bit + 1);
-      Value indBitRem = rewriter.createOrFold<CatPrimOp>(
-          op.getLoc(), indBitRemHigh, indBitRemLow);
-      Value otherSide =
-          rewriter.create<MultibitMuxOp>(op.getLoc(), indBitRem, nonSimple);
-      Value high = curval ? otherSide : v;
-      Value low = curval ? v : otherSide;
-      replaceOpWithNewOpAndCopyName<MuxPrimOp>(rewriter, op, indBit, high, low);
-      return success();
-    }
-  }
-
   // If the size is 2, canonicalize into a normal mux to introduce more folds.
   if (op.getInputs().size() != 2)
     return failure();
diff --git a/test/Dialect/FIRRTL/canonicalization.mlir b/test/Dialect/FIRRTL/canonicalization.mlir
index 25ab5a5d8b1e..6587c21a017f 100644
--- a/test/Dialect/FIRRTL/canonicalization.mlir
+++ b/test/Dialect/FIRRTL/canonicalization.mlir
@@ -3309,13 +3309,4 @@ firrtl.module @Whens(in %clock: !firrtl.clock, in %a: !firrtl.uint<1>, in %reset
   }
 }
 
-// CHECK-LABEL: firrtl.module @UselessIndexBit
-firrtl.module @UselessIndexBit(in %a: !firrtl.uint<3>, out %b: !firrtl.uint<4>, in %c: !firrtl.uint<4>, in %d: !firrtl.uint<4>, in %e: !firrtl.uint<4>) attributes {convention = #firrtl<convention scalarized>} {
-  %c0_ui4 = firrtl.constant 0 : !firrtl.uint<4> {name = "ttable_2"}
-  %0 = firrtl.multibit_mux %a, %c0_ui4, %c0_ui4, %c0_ui4, %c0_ui4, %c0_ui4, %c0_ui4, %d, %c : !firrtl.uint<3>, !firrtl.uint<4>
-  firrtl.strictconnect %b, %0 : !firrtl.uint<4>
-  // CHECK: firrtl.mux({{.*}}, %d, %c)
-  // CHECK: firrtl.mux({{.*}}, %c0_ui4, {{.*}})
-}
-
 }