[ValueTracking] return zero when there's conflict in known bits of a shift (PR34838)

Poison allows us to return a better result than undef.

llvm-svn: 315595

3 files changed, 19 insertions, 42 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d9e164500c3..afdef2822ac 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -810,19 +810,20 @@ static void computeKnownBitsFromShiftOperator(
     computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
     Known.Zero = KZF(Known.Zero, ShiftAmt);
     Known.One  = KOF(Known.One, ShiftAmt);
-    // If there is conflict between Known.Zero and Known.One, this must be an
-    // overflowing left shift, so the shift result is undefined. Clear Known
-    // bits so that other code could propagate this undef.
-    if ((Known.Zero & Known.One) != 0)
-      Known.resetAll();
+    // 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();
 
     return;
   }
 
   computeKnownBits(I->getOperand(1), Known, Depth + 1, Q);
 
-  // If the shift amount could be greater than or equal to the bit-width of the LHS, the
-  // value could be undef, so we don't know anything about it.
+  // 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 ((~Known.Zero).uge(BitWidth)) {
     Known.resetAll();
     return;
@@ -878,13 +879,10 @@ static void computeKnownBitsFromShiftOperator(
     Known.One  &= KOF(Known2.One, ShiftAmt);
   }
 
-  // If there are no compatible shift amounts, then we've proven that the shift
-  // amount must be >= the BitWidth, and the result is undefined. We could
-  // return anything we'd like, but we need to make sure the sets of known bits
-  // stay disjoint (it should be better for some other code to actually
-  // propagate the undef than to pick a value here using known bits).
-  if (Known.Zero.intersects(Known.One))
-    Known.resetAll();
+  // If the known bits conflict, the result is poison. Return a 0 and hope the
+  // caller can further optimize that.
+  if (Known.hasConflict())
+    Known.setAllZero();
 }
 
 static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
diff --git a/llvm/test/Analysis/ValueTracking/known-signbit-shift.ll b/llvm/test/Analysis/ValueTracking/known-signbit-shift.ll
index bf984cb7474..7e9f1c2e70c 100644
--- a/llvm/test/Analysis/ValueTracking/known-signbit-shift.ll
+++ b/llvm/test/Analysis/ValueTracking/known-signbit-shift.ll
@@ -27,28 +27,22 @@ define i1 @test_shift_negative(i32 %a, i32 %b) {
 }
 
 ; If sign bit is a known zero, it cannot be a known one.
-; This test should not crash opt.
+; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict1(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict1(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 %b, i32 -2147221504, i32 -2147483648
-; CHECK-NEXT:    ret i32 [[SEL]]
+; CHECK-NEXT:    ret i32 0
 ;
-entry:
   %sel = select i1 %b, i32 8193, i32 8192
   %mul = shl nsw i32 %sel, 18
   ret i32 %mul
 }
 
 ; If sign bit is a known one, it cannot be a known zero.
-; This test should not crash opt.
+; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict2(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict2(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 %b, i32 2147221504, i32 2146959360
-; CHECK-NEXT:    ret i32 [[SEL]]
+; CHECK-NEXT:    ret i32 0
 ;
-entry:
   %sel = select i1 %b, i32 -8193, i32 -8194
   %mul = shl nsw i32 %sel, 18
   ret i32 %mul
diff --git a/llvm/test/Transforms/InstSimplify/icmp-constant.ll b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
index 453d4befb92..2e58799f970 100644
--- a/llvm/test/Transforms/InstSimplify/icmp-constant.ll
+++ b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
@@ -576,12 +576,7 @@ define <2 x i1> @add_nsw_pos_const5_splat_vec(<2 x i32> %x) {
 
 define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @ne_shl_by_constant_produces_poison(
-; 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]]
+; CHECK-NEXT:    ret i1 true
 ;
   %zx = zext i8 %x to i16      ; zx  = 0x00xx
   %xor = xor i16 %zx, 32767    ; xor = 0x7fyy
@@ -593,11 +588,7 @@ define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 
 define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @eq_shl_by_constant_produces_poison(
-; 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]]
+; CHECK-NEXT:    ret i1 false
 ;
   %clear_high_bit = and i8 %x, 127                 ; 0x7f
   %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
@@ -612,13 +603,7 @@ define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
 
 define i1 @eq_shl_by_variable_produces_poison(i8 %x) {
 ; CHECK-LABEL: @eq_shl_by_variable_produces_poison(
-; CHECK-NEXT:    [[CLEAR_HIGH_BIT:%.*]] = and i8 %x, 127
-; CHECK-NEXT:    [[SET_NEXT_HIGH_BITS:%.*]] = or i8 [[CLEAR_HIGH_BIT]], 112
-; CHECK-NEXT:    [[NOTUNDEF_SHIFTAMT:%.*]] = and i8 %x, 3
-; CHECK-NEXT:    [[NONZERO_SHIFTAMT:%.*]] = or i8 [[NOTUNDEF_SHIFTAMT]], 1
-; CHECK-NEXT:    [[POISON:%.*]] = shl nsw i8 [[SET_NEXT_HIGH_BITS]], [[NONZERO_SHIFTAMT]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[POISON]], 15
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   %clear_high_bit = and i8 %x, 127                 ; 0x7f
   %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70