[SelectionDAG]: Deduce known bits from SMIN and SMAX

Author: AZero13

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5360,10 +5360,20 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op, unsigned Depth) const {
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) ||
            isKnownNeverZero(Op.getOperand(0), Depth + 1);
 
-    // TODO for smin/smax: If either operand is known negative/positive
+    // For smin/smax: If either operand is known negative/positive
     // respectively we don't need the other to be known at all.
   case ISD::SMAX:
+    if (computeKnownBits(Op.getOperand(1), Depth + 1).isStrictlyPositive() ||
+        computeKnownBits(Op.getOperand(0), Depth + 1).isStrictlyPositive())
+      return true;
+    return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
+           isKnownNeverZero(Op.getOperand(0), Depth + 1);
   case ISD::SMIN:
+    if (computeKnownBits(Op.getOperand(1), Depth + 1).isNegative() ||
+        computeKnownBits(Op.getOperand(0), Depth + 1).isNegative())
+      return true;
+    return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
+           isKnownNeverZero(Op.getOperand(0), Depth + 1);
   case ISD::UMIN:
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
            isKnownNeverZero(Op.getOperand(0), Depth + 1);

llvm/test/CodeGen/X86/known-never-zero.ll

@@ -273,9 +273,7 @@ define i32 @smin_known_zero_2(i32 %x, i32 %y) {
 ; CHECK-NEXT:    cmpl $-54, %edi
 ; CHECK-NEXT:    movl $-54, %eax
 ; CHECK-NEXT:    cmovll %edi, %eax
-; CHECK-NEXT:    bsfl %eax, %ecx
-; CHECK-NEXT:    movl $32, %eax
-; CHECK-NEXT:    cmovnel %ecx, %eax
+; CHECK-NEXT:    rep bsfl %eax, %eax
 ; CHECK-NEXT:    retq
   %z = call i32 @llvm.smin.i32(i32 %x, i32 -54)
   %r = call i32 @llvm.cttz.i32(i32 %z, i1 false)
@@ -326,9 +324,7 @@ define i32 @smax_known_zero_2(i32 %x, i32 %y) {
 ; CHECK-NEXT:    cmpl $55, %edi
 ; CHECK-NEXT:    movl $54, %eax
 ; CHECK-NEXT:    cmovgel %edi, %eax
-; CHECK-NEXT:    bsfl %eax, %ecx
-; CHECK-NEXT:    movl $32, %eax
-; CHECK-NEXT:    cmovnel %ecx, %eax
+; CHECK-NEXT:    rep bsfl %eax, %eax
 ; CHECK-NEXT:    retq
   %z = call i32 @llvm.smax.i32(i32 %x, i32 54)
   %r = call i32 @llvm.cttz.i32(i32 %z, i1 false)