[PATCH] D156777: [SelectionDAG] Add/Improve cases in `isKnownNeverZero`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n updated this revision to Diff 547250.
goldstein.w.n marked an inline comment as done.
goldstein.w.n added a comment.

Fix shift cnt check


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D156777/new/

https://reviews.llvm.org/D156777

Files:
  llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  llvm/test/CodeGen/X86/known-never-zero.ll


Index: llvm/test/CodeGen/X86/known-never-zero.ll
===================================================================
--- llvm/test/CodeGen/X86/known-never-zero.ll
+++ llvm/test/CodeGen/X86/known-never-zero.ll
@@ -258,9 +258,7 @@
 ; CHECK-NEXT:    addl $4, %esi
 ; CHECK-NEXT:    cmpl %esi, %eax
 ; CHECK-NEXT:    cmovll %eax, %esi
-; CHECK-NEXT:    bsfl %esi, %ecx
-; CHECK-NEXT:    movl $32, %eax
-; CHECK-NEXT:    cmovnel %ecx, %eax
+; CHECK-NEXT:    rep bsfl %esi, %eax
 ; CHECK-NEXT:    retq
   %x = shl nuw i32 4, %xx
   %y = add nuw nsw i32 %yy, 4
@@ -298,9 +296,7 @@
 ; CHECK-NEXT:    addl $4, %esi
 ; CHECK-NEXT:    cmpl %esi, %eax
 ; CHECK-NEXT:    cmovgl %eax, %esi
-; CHECK-NEXT:    bsfl %esi, %ecx
-; CHECK-NEXT:    movl $32, %eax
-; CHECK-NEXT:    cmovnel %ecx, %eax
+; CHECK-NEXT:    rep bsfl %esi, %eax
 ; CHECK-NEXT:    retq
   %x = shl nuw i32 4, %xx
   %y = add nuw nsw i32 %yy, 4
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5105,21 +5105,29 @@
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
            isKnownNeverZero(Op.getOperand(2), Depth + 1);
 
-  case ISD::SHL:
+  case ISD::SHL: {
     if (Op->getFlags().hasNoSignedWrap() || Op->getFlags().hasNoUnsignedWrap())
       return isKnownNeverZero(Op.getOperand(0), Depth + 1);
-
-    // 1 << X is never zero. TODO: This can be expanded if we can bound X.
-    // The expression is really !Known.One[BitWidth-MaxLog2(Known):0].isZero()
-    if (computeKnownBits(Op.getOperand(0), Depth + 1).One[0])
+    KnownBits ValKnown = computeKnownBits(Op.getOperand(0), Depth + 1);
+    // 1 << X is never zero.
+    if (ValKnown.One[0])
+      return true;
+    // If max shift cnt of known ones is non-zero, result is non-zero.
+    APInt MaxCnt = computeKnownBits(Op.getOperand(1), Depth + 1).getMaxValue();
+    if (MaxCnt.ult(ValKnown.getBitWidth()) &&
+        !ValKnown.One.shl(MaxCnt).isZero())
       return true;
     break;
-
+  }
   case ISD::UADDSAT:
   case ISD::UMAX:
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) ||
            isKnownNeverZero(Op.getOperand(0), Depth + 1);
 
+    // TODO 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:
+  case ISD::SMIN:
   case ISD::UMIN:
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
            isKnownNeverZero(Op.getOperand(0), Depth + 1);
@@ -5133,16 +5141,19 @@
     return isKnownNeverZero(Op.getOperand(0), Depth + 1);
 
   case ISD::SRA:
-  case ISD::SRL:
+  case ISD::SRL: {
     if (Op->getFlags().hasExact())
       return isKnownNeverZero(Op.getOperand(0), Depth + 1);
-    // Signed >> X is never zero. TODO: This can be expanded if we can bound X.
-    // The expression is really
-    // !Known.One[SignBit:SignBit-(BitWidth-MaxLog2(Known))].isZero()
-    if (computeKnownBits(Op.getOperand(0), Depth + 1).isNegative())
+    KnownBits ValKnown = computeKnownBits(Op.getOperand(0), Depth + 1);
+    if (ValKnown.isNegative())
+      return true;
+    // If max shift cnt of known ones is non-zero, result is non-zero.
+    APInt MaxCnt = computeKnownBits(Op.getOperand(1), Depth + 1).getMaxValue();
+    if (MaxCnt.ult(ValKnown.getBitWidth()) &&
+        !ValKnown.One.lshr(MaxCnt).isZero())
       return true;
     break;
-
+  }
   case ISD::UDIV:
   case ISD::SDIV:
     // div exact can only produce a zero if the dividend is zero.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D156777.547250.patch
Type: text/x-patch
Size: 3588 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230804/baf66c79/attachment.bin>