[llvm] r315595 - [ValueTracking] return zero when there's conflict in known bits of a shift (PR34838)
Sanjay Patel via llvm-commits
llvm-commits at lists.llvm.org
Thu Oct 12 10:31:47 PDT 2017
Author: spatel
Date: Thu Oct 12 10:31:46 2017
New Revision: 315595
URL: http://llvm.org/viewvc/llvm-project?rev=315595&view=rev
Log:
[ValueTracking] return zero when there's conflict in known bits of a shift (PR34838)
Poison allows us to return a better result than undef.
Modified:
llvm/trunk/lib/Analysis/ValueTracking.cpp
llvm/trunk/test/Analysis/ValueTracking/known-signbit-shift.ll
llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll
Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=315595&r1=315594&r2=315595&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Thu Oct 12 10:31:46 2017
@@ -810,19 +810,20 @@ static void computeKnownBitsFromShiftOpe
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 computeKnownBitsFromShiftOpe
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,
Modified: llvm/trunk/test/Analysis/ValueTracking/known-signbit-shift.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ValueTracking/known-signbit-shift.ll?rev=315595&r1=315594&r2=315595&view=diff
==============================================================================
--- llvm/trunk/test/Analysis/ValueTracking/known-signbit-shift.ll (original)
+++ llvm/trunk/test/Analysis/ValueTracking/known-signbit-shift.ll Thu Oct 12 10:31:46 2017
@@ -27,28 +27,22 @@ define i1 @test_shift_negative(i32 %a, i
}
; 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
Modified: llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll?rev=315595&r1=315594&r2=315595&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/icmp-constant.ll Thu Oct 12 10:31:46 2017
@@ -576,12 +576,7 @@ define <2 x i1> @add_nsw_pos_const5_spla
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_p
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_p
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
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