[PATCH] D21078: [Thumb] Select a BIC instead of AND if the immediate can be encoded more optimally negated

[James Molloy via llvm-commits]

jmolloy updated this revision to Diff 60028.
jmolloy added a comment.

Thanks Chad and Silviu for the review.

Updated according to Chad's comments, and hopefully the new name "PreferImmediateEncoding" should address Silviu's comment too.


Repository:
  rL LLVM

http://reviews.llvm.org/D21078

Files:
  lib/Target/ARM/ARMISelDAGToDAG.cpp
  test/CodeGen/Thumb/bic_imm.ll

Index: test/CodeGen/Thumb/bic_imm.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Thumb/bic_imm.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -mtriple=thumbv7-linux-gnueabi -mcpu=cortex-m0 -verify-machineinstrs | FileCheck --check-prefix CHECK-T1 %s
+; RUN: llc < %s -mtriple=thumbv7-linux-gnueabi -mcpu=cortex-m3 -verify-machineinstrs | FileCheck --check-prefix CHECK-T2 %s
+
+; CHECK-T1-LABEL: @i
+; CHECK-T2-LABEL: @i
+; CHECK-T1: movs r1, #255
+; CHECK-T1: adds r1, #20
+; CHECK-T1: bics r0, r1
+; CHECK-T2: movw r1, #275
+; CHECK-T2: bics r0, r1
+define i32 @i(i32 %a) {
+entry:
+  %and = and i32 %a, -276
+  ret i32 %and
+}
Index: lib/Target/ARM/ARMISelDAGToDAG.cpp
===================================================================
--- lib/Target/ARM/ARMISelDAGToDAG.cpp
+++ lib/Target/ARM/ARMISelDAGToDAG.cpp
@@ -2797,6 +2797,45 @@
     if (tryV6T2BitfieldExtractOp(N, false))
       return;
 
+    // If an immediate is used in an AND node, it is possible that the immediate
+    // can be more optimally materialized when negated. If this is the case we
+    // can negate the immediate and use a BIC instead.
+    auto *N1C = dyn_cast<ConstantSDNode>(N->getOperand(1));
+    if (N1C && N1C->hasOneUse() && Subtarget->isThumb()) {
+      uint32_t Imm = (uint32_t) N1C->getZExtValue();
+
+      // In Thumb2 mode, an AND can take a 12-bit immediate. If this
+      // immediate can be negated and fit in the immediate operand of
+      // a t2BIC, don't do any manual transform here as this can be
+      // handled by the generic ISel machinery.
+      bool PreferImmediateEncoding =
+          Subtarget->hasThumb2() && !is_t2_so_imm(Imm) && is_t2_so_imm_not(Imm);
+      if (!PreferImmediateEncoding &&
+          ConstantMaterializationCost(Imm) >
+              ConstantMaterializationCost(~Imm)) {
+        // The current immediate costs more to materialize than a negated
+        // immediate, so negate the immediate and use a BIC.
+        SDValue NewImm =
+            CurDAG->getConstant(~N1C->getZExtValue(), dl, MVT::i32);
+        CurDAG->RepositionNode(N->getIterator(), NewImm.getNode());
+
+        if (!Subtarget->hasThumb2()) {
+          SDValue Ops[] = {CurDAG->getRegister(ARM::CPSR, MVT::i32),
+                           N->getOperand(0), NewImm, getAL(CurDAG, dl),
+                           CurDAG->getRegister(0, MVT::i32)};
+          ReplaceNode(N, CurDAG->getMachineNode(ARM::tBIC, dl, MVT::i32, Ops));
+          return;
+        } else {
+          SDValue Ops[] = {N->getOperand(0), NewImm, getAL(CurDAG, dl),
+                           CurDAG->getRegister(0, MVT::i32),
+                           CurDAG->getRegister(0, MVT::i32)};
+          ReplaceNode(N,
+                      CurDAG->getMachineNode(ARM::t2BICrr, dl, MVT::i32, Ops));
+          return;
+        }
+      }
+    }
+
     // (and (or x, c2), c1) and top 16-bits of c1 and c2 match, lower 16-bits
     // of c1 are 0xffff, and lower 16-bit of c2 are 0. That is, the top 16-bits
     // are entirely contributed by c2 and lower 16-bits are entirely contributed
@@ -2811,7 +2850,7 @@
     if (!Opc)
       break;
     SDValue N0 = N->getOperand(0), N1 = N->getOperand(1);
-    ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1);
+    N1C = dyn_cast<ConstantSDNode>(N1);
     if (!N1C)
       break;
     if (N0.getOpcode() == ISD::OR && N0.getNode()->hasOneUse()) {


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