[PATCH] D90479: [ValueTracking] ComputeKnownBits - minimum leading/trailing zero bits in LSHR/SHL (PR44526)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon updated this revision to Diff 325446.
RKSimon added a comment.

Rebase + simplify after D95959 <https://reviews.llvm.org/D95959>

Poison nsw shl patterns no longer return zero and instead bails out - InstSimplify should handle this and return poison.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90479/new/

https://reviews.llvm.org/D90479

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstSimplify/icmp-constant.ll


Index: llvm/test/Transforms/InstSimplify/icmp-constant.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/icmp-constant.ll
+++ llvm/test/Transforms/InstSimplify/icmp-constant.ll
@@ -772,7 +772,12 @@
 
 define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @ne_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[ZX:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[XOR:%.*]] = xor i16 [[ZX]], 32767
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i16 [[ZX]], [[XOR]]
+; CHECK-NEXT:    [[POISON:%.*]] = shl nsw i16 [[SUB]], 2
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i16 [[POISON]], 1
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %zx = zext i8 %x to i16      ; zx  = 0x00xx
   %xor = xor i16 %zx, 32767    ; xor = 0x7fyy
@@ -784,7 +789,11 @@
 
 define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @eq_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    [[CLEAR_HIGH_BIT:%.*]] = and i8 [[X:%.*]], 127
+; CHECK-NEXT:    [[SET_NEXT_HIGH_BITS:%.*]] = or i8 [[CLEAR_HIGH_BIT]], 112
+; CHECK-NEXT:    [[POISON:%.*]] = shl nsw i8 [[SET_NEXT_HIGH_BITS]], 3
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[POISON]], 15
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %clear_high_bit = and i8 %x, 127                 ; 0x7f
   %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -994,30 +994,25 @@
   bool ShiftAmtIsConstant = Known.isConstant();
   bool MaxShiftAmtIsOutOfRange = Known.getMaxValue().uge(BitWidth);
 
-  if (ShiftAmtIsConstant) {
-    Known = KF(Known2, Known);
-
-    // If the known bits conflict, this must be an overflowing left shift, so
-    // the shift result is poison. We can return anything we want. Choose 0 for
-    // the best folding opportunity.
-    if (Known.hasConflict())
-      Known.setAllZero();
+  // Use the KF callback to get an initial knownbits approximation.
+  Known = KF(Known2, Known);
 
+  // If the known bits conflict, this must be an overflowing left shift, so
+  // the shift result is poison.
+  if (Known.hasConflict()) {
+    Known.resetAll();
     return;
   }
 
+  if (ShiftAmtIsConstant)
+    return;
+
   // If the shift amount could be greater than or equal to the bit-width of the
   // LHS, the value could be poison, but bail out because the check below is
   // expensive.
   // TODO: Should we just carry on?
-  if (MaxShiftAmtIsOutOfRange) {
-    Known.resetAll();
+  if (MaxShiftAmtIsOutOfRange)
     return;
-  }
-
-  // It would be more-clearly correct to use the two temporaries for this
-  // calculation. Reusing the APInts here to prevent unnecessary allocations.
-  Known.resetAll();
 
   // If we know the shifter operand is nonzero, we can sometimes infer more
   // known bits. However this is expensive to compute, so be lazy about it and
@@ -1232,10 +1227,6 @@
     };
     computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
                                       KF);
-    // Trailing zeros of a right-shifted constant never decrease.
-    const APInt *C;
-    if (match(I->getOperand(0), m_APInt(C)))
-      Known.Zero.setLowBits(C->countTrailingZeros());
     break;
   }
   case Instruction::LShr: {
@@ -1244,10 +1235,6 @@
     };
     computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
                                       KF);
-    // Leading zeros of a left-shifted constant never decrease.
-    const APInt *C;
-    if (match(I->getOperand(0), m_APInt(C)))
-      Known.Zero.setHighBits(C->countLeadingZeros());
     break;
   }
   case Instruction::AShr: {


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