Teach the codegen to turn [aez]ext (setcc) -> selectcc of 1/0, which often

allows other simplifications.  For example, this compiles:
int isnegative(unsigned int X) {
   return !(X < 2147483648U);
}

Into this code:

x86:
        movl 4(%esp), %eax
        shrl $31, %eax
        ret
arm:
        mov r0, r0, lsr #31
        bx lr
thumb:
        lsr r0, r0, #31
        bx lr

instead of:

x86:
        cmpl $0, 4(%esp)
        sets %al
        movzbl %al, %eax
        ret

arm:
        mov r3, #0
        cmp r0, #0
        movlt r3, #1
        mov r0, r3
        bx lr

thumb:
        mov r2, #1
        mov r1, #0
        cmp r0, #0
        blt LBB1_2      @entry
LBB1_1: @entry
        cpy r2, r1
LBB1_2: @entry
        cpy r0, r2
        bx lr

Testcase here: test/CodeGen/Generic/ispositive.ll

llvm-svn: 35883

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -2130,6 +2130,15 @@ SDOperand DAGCombiner::visitSIGN_EXTEND(SDNode *N) {
     }
   }
   
+  // sext(setcc x,y,cc) -> select_cc x, y, -1, 0, cc
+  if (N0.getOpcode() == ISD::SETCC) {
+    SDOperand SCC = 
+    SimplifySelectCC(N0.getOperand(0), N0.getOperand(1),
+                     DAG.getConstant(~0ULL, VT), DAG.getConstant(0, VT),
+                     cast<CondCodeSDNode>(N0.getOperand(2))->get());
+    if (SCC.Val) return SCC;
+  }
+  
   return SDOperand();
 }
 
@@ -2210,6 +2219,16 @@ SDOperand DAGCombiner::visitZERO_EXTEND(SDNode *N) {
               ExtLoad.getValue(1));
     return SDOperand(N, 0);   // Return N so it doesn't get rechecked!
   }
+  
+  // zext(setcc x,y,cc) -> select_cc x, y, 1, 0, cc
+  if (N0.getOpcode() == ISD::SETCC) {
+    SDOperand SCC = 
+      SimplifySelectCC(N0.getOperand(0), N0.getOperand(1),
+                       DAG.getConstant(1, VT), DAG.getConstant(0, VT),
+                       cast<CondCodeSDNode>(N0.getOperand(2))->get());
+    if (SCC.Val) return SCC;
+  }
+  
   return SDOperand();
 }
 
@@ -2294,6 +2313,16 @@ SDOperand DAGCombiner::visitANY_EXTEND(SDNode *N) {
               ExtLoad.getValue(1));
     return SDOperand(N, 0);   // Return N so it doesn't get rechecked!
   }
+  
+  // aext(setcc x,y,cc) -> select_cc x, y, 1, 0, cc
+  if (N0.getOpcode() == ISD::SETCC) {
+    SDOperand SCC = 
+    SimplifySelectCC(N0.getOperand(0), N0.getOperand(1),
+                     DAG.getConstant(1, VT), DAG.getConstant(0, VT),
+                     cast<CondCodeSDNode>(N0.getOperand(2))->get());
+    if (SCC.Val) return SCC;
+  }
+  
   return SDOperand();
 }