[PATCH] D147897: [ValueTracking] Use maximum shift count in `shl` when determining if `shl` can be zero.

[Noah Goldstein via Phabricator via llvm-commits]

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Previously only return `shl` non-zero if the shift value was `1`. We
can expand this if we have some bounds on the shift count.

For example:

  %cnt = and %c, 16 ; Max cnt == 16
  %val = or %v, 4 ; val[2] is known one
  %shl = shl %val, %cnt ; (val.known.one << cnt.maxval) != 0



Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147897

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-non-zero.ll


Index: llvm/test/Analysis/ValueTracking/known-non-zero.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -170,11 +170,7 @@
 
 define <2 x i1> @shl_nz_bounded_cnt_vec(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @shl_nz_bounded_cnt_vec(
-; CHECK-NEXT:    [[CNT:%.*]] = and <2 x i32> [[X:%.*]], <i32 16, i32 24>
-; CHECK-NEXT:    [[VAL:%.*]] = or <2 x i32> [[Y:%.*]], <i32 131088, i32 16>
-; CHECK-NEXT:    [[SHL:%.*]] = shl <2 x i32> [[VAL]], [[CNT]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq <2 x i32> [[SHL]], zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[R]]
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
 ;
   %cnt = and <2 x i32> %x, <i32 16, i32 24>
   %val = or <2 x i32> %y, <i32 131088, i32 16>
@@ -187,10 +183,7 @@
 ; CHECK-LABEL: @shl_nz_bounded_cnt(
 ; CHECK-NEXT:    [[CNT_ULT4:%.*]] = icmp ult i32 [[CNT:%.*]], 4
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[CNT_ULT4]])
-; CHECK-NEXT:    [[VAL:%.*]] = or i32 [[Y:%.*]], 131072
-; CHECK-NEXT:    [[SHL:%.*]] = shl i32 [[VAL]], [[CNT]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i32 [[SHL]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %cnt_ult4 = icmp ult i32 %cnt, 4
   call void @llvm.assume(i1 %cnt_ult4)
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2681,6 +2681,14 @@
     computeKnownBits(I->getOperand(0), DemandedElts, Known, Depth, Q);
     if (Known.One[0])
       return true;
+
+    if (!Known.isUnknown()) {
+      KnownBits KnownCnt =
+          computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
+      if (!Known.One.shl(KnownCnt.getMaxValue()).isZero())
+        return true;
+    }
+
     break;
   }
   case Instruction::LShr:


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