Target independent DAG transform to use truncate for field extraction + sign extend on targets where this is profitable. Passes nightly on x86-64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48491 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 65 insertions, 0 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index acc00fd58b..737d9661fd 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2374,6 +2374,32 @@ SDOperand DAGCombiner::visitSRA(SDNode *N) {
                          DAG.getValueType(EVT));
   }
   
+  // fold sra (shl X, m), result_size - n 
+  // -> (sign_extend (trunc (shl X, result_size - n - m))) for 
+  // result_size - n != m. If truncate is free for the target sext(shl) is 
+  // likely to result in better code.
+  if (N0.getOpcode() == ISD::SHL) {
+    // Get the two constanst of the shifts, CN0 = m, CN = n.
+    const ConstantSDNode *N01C = dyn_cast<ConstantSDNode>(N0.getOperand(1));
+    if (N01C && N1C) {
+      // Determine if the truncate type's bitsize would correspond to
+      // an integer type for this target.
+      unsigned VTValSize = MVT::getSizeInBits(VT);
+      MVT::ValueType TruncVT = MVT::getIntegerType(VTValSize - N1C->getValue());
+      unsigned ShiftAmt = N1C->getValue() - N01C->getValue();
+      
+      // If the shift wouldn't be a noop, the truncated type is an actual type, 
+      // and the truncate is free, then proceed with the transform.
+      if (ShiftAmt != 0 && 
+          !MVT::isExtendedVT(TruncVT) && TLI.isTruncateFree(VT, TruncVT)) {
+        SDOperand Amt = DAG.getConstant(ShiftAmt, TLI.getShiftAmountTy());
+        SDOperand Shift = DAG.getNode(ISD::SRL, VT, N0.getOperand(0), Amt);
+        SDOperand Trunc = DAG.getNode(ISD::TRUNCATE, TruncVT, Shift);
+        return DAG.getNode(ISD::SIGN_EXTEND, N->getValueType(0), Trunc);
+      }
+    }
+  }
+  
   // fold (sra (sra x, c1), c2) -> (sra x, c1+c2)
   if (N1C && N0.getOpcode() == ISD::SRA) {
     if (ConstantSDNode *C1 = dyn_cast<ConstantSDNode>(N0.getOperand(1))) {
diff --git a/test/CodeGen/X86/field-extract-use-trunc.ll b/test/CodeGen/X86/field-extract-use-trunc.ll
new file mode 100644
index 0000000000..63172b6bf7
--- /dev/null
+++ b/test/CodeGen/X86/field-extract-use-trunc.ll
@@ -0,0 +1,39 @@
+; RUN: llvm-as < %s | llc -march=x86 | grep sar | count 1
+; RUN: llvm-as < %s | llc -march=x86-64 | not grep sar
+
+define i32 @test(i32 %f12) {
+	%tmp7.25 = lshr i32 %f12, 16		
+	%tmp7.26 = trunc i32 %tmp7.25 to i8
+	%tmp78.2 = sext i8 %tmp7.26 to i32
+	ret i32 %tmp78.2
+}
+
+define i32 @test2(i32 %f12) {
+	%f11 = shl i32 %f12, 8
+	%tmp7.25 = ashr i32 %f11, 24
+	ret i32 %tmp7.25
+}
+
+define i32 @test3(i32 %f12) {
+	%f11 = shl i32 %f12, 13
+	%tmp7.25 = ashr i32 %f11, 24
+	ret i32 %tmp7.25
+}
+
+define i64 @test4(i64 %f12) {
+	%f11 = shl i64 %f12, 32
+	%tmp7.25 = ashr i64 %f11, 32
+	ret i64 %tmp7.25
+}
+
+define i16 @test5(i16 %f12) {
+	%f11 = shl i16 %f12, 2
+	%tmp7.25 = ashr i16 %f11, 8
+	ret i16 %tmp7.25
+}
+
+define i16 @test6(i16 %f12) {
+	%f11 = shl i16 %f12, 8
+	%tmp7.25 = ashr i16 %f11, 8
+	ret i16 %tmp7.25
+}
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