Emit a more efficient magic number multiplication for exact sdivs.

We have to do this in DAGBuilder instead of DAGCombiner, because the exact bit is lost after building.

  struct foo { char x[24]; };
  long bar(struct foo *a, struct foo *b) { return a-b; }
is now compiled into
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  sarl	$3, %eax
  imull	$-1431655765, %eax, %eax
instead of
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  movl	$715827883, %ecx
  imull	%ecx
  movl	%edx, %eax
  shrl	$31, %eax
  sarl	$2, %edx
  addl	%eax, %edx
  movl	%edx, %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134695 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index fc8f0d2996..2a362ebfc2 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2501,6 +2501,22 @@ void SelectionDAGBuilder::visitShift(const User &I, unsigned Opcode) {
                            Op1.getValueType(), Op1, Op2));
 }
 
+void SelectionDAGBuilder::visitSDiv(const User &I) {
+  const BinaryOperator *BO = cast<BinaryOperator>(&I);
+  SDValue Op1 = getValue(I.getOperand(0));
+  SDValue Op2 = getValue(I.getOperand(1));
+
+  // Turn exact SDivs into multiplications.
+  // FIXME: This should be in DAGCombiner, but it doesn't have access to the
+  // exact bit.
+  if (BO->isExact() && !isa<ConstantSDNode>(Op1) &&
+      isa<ConstantSDNode>(Op2) && !cast<ConstantSDNode>(Op2)->isNullValue())
+    setValue(&I, TLI.BuildExactSDIV(Op1, Op2, getCurDebugLoc(), DAG));
+  else
+    setValue(&I, DAG.getNode(ISD::SDIV, getCurDebugLoc(), Op1.getValueType(),
+                             Op1, Op2));
+}
+
 void SelectionDAGBuilder::visitICmp(const User &I) {
   ICmpInst::Predicate predicate = ICmpInst::BAD_ICMP_PREDICATE;
   if (const ICmpInst *IC = dyn_cast<ICmpInst>(&I))