[llvm] 730df5a - [Support] Add KnownBits::computeForSubBorrow (#67788)
via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 18 05:48:51 PDT 2023
Author: Christian Kissig
Date: 2023-10-18T13:48:47+01:00
New Revision: 730df5a437914e66b292dd9cb7d5f3f47b73bab0
URL: https://github.com/llvm/llvm-project/commit/730df5a437914e66b292dd9cb7d5f3f47b73bab0
DIFF: https://github.com/llvm/llvm-project/commit/730df5a437914e66b292dd9cb7d5f3f47b73bab0.diff
LOG: [Support] Add KnownBits::computeForSubBorrow (#67788)
- [Support] Add KnownBits::computeForSubBorrow
- [CodeGen] Implement USUBC, USUBO_CARRY, and SSUBO_CARRY with
KnownBits::computeForSubBorrow
- [CodeGen] Compute unknown bits for Carry/Borrow for ADD/SUB
- [CodeGen] Compute known bits of Carry/Borrow for UADDO, SADDO, USUBO,
and SSUBO
Fixes #65893
---------
Co-authored-by: Shafik Yaghmour <shafik at users.noreply.github.com>
Added:
Modified:
llvm/include/llvm/Support/KnownBits.h
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
llvm/lib/Support/KnownBits.cpp
llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
llvm/unittests/Support/KnownBitsTest.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index 8462aa11202d5d7..fb034e0b9e3baff 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -332,6 +332,11 @@ struct KnownBits {
static KnownBits computeForAddSub(bool Add, bool NSW, const KnownBits &LHS,
KnownBits RHS);
+ /// Compute known bits results from subtracting RHS from LHS with 1-bit
+ /// Borrow.
+ static KnownBits computeForSubBorrow(const KnownBits &LHS, KnownBits RHS,
+ const KnownBits &Borrow);
+
/// Compute knownbits resulting from llvm.sadd.sat(LHS, RHS)
static KnownBits sadd_sat(const KnownBits &LHS, const KnownBits &RHS);
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 8c275bfcfbd2796..3c131d9247d7240 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3741,14 +3741,19 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
assert(Op.getResNo() == 0 &&
"We only compute knownbits for the
diff erence here.");
- // TODO: Compute influence of the carry operand.
- if (Opcode == ISD::USUBO_CARRY || Opcode == ISD::SSUBO_CARRY)
- break;
+ // With USUBO_CARRY and SSUBO_CARRY a borrow bit may be added in.
+ KnownBits Borrow(1);
+ if (Opcode == ISD::USUBO_CARRY || Opcode == ISD::SSUBO_CARRY) {
+ Borrow = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
+ // Borrow has bit width 1
+ Borrow = Borrow.trunc(1);
+ } else {
+ Borrow.setAllZero();
+ }
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
- Known = KnownBits::computeForAddSub(/* Add */ false, /* NSW */ false,
- Known, Known2);
+ Known = KnownBits::computeForSubBorrow(Known, Known2, Borrow);
break;
}
case ISD::UADDO:
@@ -3773,15 +3778,13 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
if (Opcode == ISD::ADDE)
// Can't track carry from glue, set carry to unknown.
Carry.resetAll();
- else if (Opcode == ISD::UADDO_CARRY || Opcode == ISD::SADDO_CARRY)
- // TODO: Compute known bits for the carry operand. Not sure if it is worth
- // the trouble (how often will we find a known carry bit). And I haven't
- // tested this very much yet, but something like this might work:
- // Carry = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
- // Carry = Carry.zextOrTrunc(1, false);
- Carry.resetAll();
- else
+ else if (Opcode == ISD::UADDO_CARRY || Opcode == ISD::SADDO_CARRY) {
+ Carry = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
+ // Carry has bit width 1
+ Carry = Carry.trunc(1);
+ } else {
Carry.setAllZero();
+ }
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 097c22d33dd12ba..770e4051ca3ffac 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -85,6 +85,18 @@ KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
return KnownOut;
}
+KnownBits KnownBits::computeForSubBorrow(const KnownBits &LHS, KnownBits RHS,
+ const KnownBits &Borrow) {
+ assert(Borrow.getBitWidth() == 1 && "Borrow must be 1-bit");
+
+ // LHS - RHS = LHS + ~RHS + 1
+ // Carry 1 - Borrow in ::computeForAddCarry
+ std::swap(RHS.Zero, RHS.One);
+ return ::computeForAddCarry(LHS, RHS,
+ /*CarryZero=*/Borrow.One.getBoolValue(),
+ /*CarryOne=*/Borrow.Zero.getBoolValue());
+}
+
KnownBits KnownBits::sextInReg(unsigned SrcBitWidth) const {
unsigned BitWidth = getBitWidth();
assert(0 < SrcBitWidth && SrcBitWidth <= BitWidth &&
diff --git a/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp b/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
index 0e1f2736907fff8..bb8e76a2eeb8beb 100644
--- a/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
+++ b/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
@@ -254,6 +254,59 @@ TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_ADD) {
EXPECT_EQ(Known.One, APInt(8, 0x55));
}
+// Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
+TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_UADDO_CARRY) {
+ SDLoc Loc;
+ auto IntVT = EVT::getIntegerVT(Context, 8);
+ auto UnknownOp = DAG->getRegister(0, IntVT);
+ auto Mask_Zero = DAG->getConstant(0x28, Loc, IntVT);
+ auto Mask_One = DAG->getConstant(0x20, Loc, IntVT);
+ auto N0 = DAG->getNode(ISD::AND, Loc, IntVT, Mask_Zero, UnknownOp);
+ N0 = DAG->getNode(ISD::OR, Loc, IntVT, Mask_One, N0);
+ auto N1 = DAG->getConstant(0x65, Loc, IntVT);
+
+ KnownBits Known;
+
+ auto UnknownBorrow = DAG->getRegister(1, IntVT);
+ auto OpUnknownBorrow =
+ DAG->getNode(ISD::UADDO_CARRY, Loc, IntVT, N0, N1, UnknownBorrow);
+ // N0 = 0010?000
+ // N1 = 01100101
+ // B = ?
+ // =>
+ // Known.Zero = 01110000 (0x70)
+ // Known.One = 10000100 (0x84)
+ Known = DAG->computeKnownBits(OpUnknownBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0x70));
+ EXPECT_EQ(Known.One, APInt(8, 0x84));
+
+ auto ZeroBorrow = DAG->getConstant(0x0, Loc, IntVT);
+ auto OpZeroBorrow =
+ DAG->getNode(ISD::UADDO_CARRY, Loc, IntVT, N0, N1, ZeroBorrow);
+ // N0 = 0010?000
+ // N1 = 01100101
+ // B = 0
+ // =>
+ // Known.Zero = 01110010 (0x72)
+ // Known.One = 10000101 (0x85)
+ Known = DAG->computeKnownBits(OpZeroBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0x72));
+ EXPECT_EQ(Known.One, APInt(8, 0x85));
+
+ auto OneBorrow = DAG->getConstant(0x1, Loc, IntVT);
+ auto OpOneBorrow =
+ DAG->getNode(ISD::UADDO_CARRY, Loc, IntVT, N0, N1, OneBorrow);
+ // N0 = 0010?000
+ // N1 = 01100101
+ // B = 1
+ // =>
+ // Known.Zero = 01110001 (0x71)
+ // Known.One = 10000110 (0x86)
+ Known = DAG->computeKnownBits(OpOneBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0x71));
+ EXPECT_EQ(Known.One, APInt(8, 0x86));
+}
+
// Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_SUB) {
SDLoc Loc;
@@ -273,6 +326,60 @@ TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_SUB) {
EXPECT_EQ(Known.One, APInt(8, 0x1));
}
+// Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
+TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_USUBO_CARRY) {
+ SDLoc Loc;
+ auto IntVT = EVT::getIntegerVT(Context, 8);
+ auto N0 = DAG->getConstant(0x5a, Loc, IntVT);
+ auto UnknownOp = DAG->getRegister(0, IntVT); // ????????
+ auto Mask1_Zero = DAG->getConstant(0x8, Loc, IntVT); // 00001000
+ auto Mask1_One = DAG->getConstant(0x20, Loc, IntVT); // 00100000
+ // N1 = (???????? & 00001000) | 00100000 = 0010?000
+ auto N1 = DAG->getNode(ISD::AND, Loc, IntVT, Mask1_Zero, UnknownOp);
+ N1 = DAG->getNode(ISD::OR, Loc, IntVT, Mask1_One, N1);
+
+ KnownBits Known;
+
+ auto UnknownBorrow = DAG->getRegister(1, IntVT);
+ auto OpUnknownBorrow =
+ DAG->getNode(ISD::USUBO_CARRY, Loc, IntVT, N0, N1, UnknownBorrow);
+ // N0 = 01011010
+ // N1 = 0010?000
+ // B = ?
+ // =>
+ // Known.Zero = 11000100 (0xc4)
+ // Known.One = 00110000 (0x30)
+ Known = DAG->computeKnownBits(OpUnknownBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0xc4));
+ EXPECT_EQ(Known.One, APInt(8, 0x30));
+
+ auto ZeroBorrow = DAG->getConstant(0x0, Loc, IntVT);
+ auto OpZeroBorrow =
+ DAG->getNode(ISD::USUBO_CARRY, Loc, IntVT, N0, N1, ZeroBorrow);
+ // N0 = 01011010
+ // N1 = 0010?000
+ // B = 0
+ // =>
+ // Known.Zero = 11000101 (0xc5)
+ // Known.One = 00110010 (0x32)
+ Known = DAG->computeKnownBits(OpZeroBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0xc5));
+ EXPECT_EQ(Known.One, APInt(8, 0x32));
+
+ auto OneBorrow = DAG->getConstant(0x1, Loc, IntVT);
+ auto OpOneBorrow =
+ DAG->getNode(ISD::USUBO_CARRY, Loc, IntVT, N0, N1, OneBorrow);
+ // N0 = 01011010
+ // N1 = 0010?000
+ // B = 1
+ // =>
+ // Known.Zero = 11000110 (0xc6)
+ // Known.One = 00110001 (0x31)
+ Known = DAG->computeKnownBits(OpOneBorrow);
+ EXPECT_EQ(Known.Zero, APInt(8, 0xc6));
+ EXPECT_EQ(Known.One, APInt(8, 0x31));
+}
+
TEST_F(AArch64SelectionDAGTest, isSplatValue_Fixed_BUILD_VECTOR) {
TargetLowering TL(*TM);
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
index 9d184beea3ba9e9..c0377d45c303a11 100644
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -213,6 +213,37 @@ TEST(KnownBitsTest, AddSubExhaustive) {
TestAddSubExhaustive(false);
}
+TEST(KnownBitsTest, SubBorrowExhaustive) {
+ unsigned Bits = 4;
+ ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
+ ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
+ ForeachKnownBits(1, [&](const KnownBits &KnownBorrow) {
+ // Explicitly compute known bits of the subtraction by trying all
+ // possibilities.
+ KnownBits Known(Bits);
+ Known.Zero.setAllBits();
+ Known.One.setAllBits();
+ ForeachNumInKnownBits(Known1, [&](const APInt &N1) {
+ ForeachNumInKnownBits(Known2, [&](const APInt &N2) {
+ ForeachNumInKnownBits(KnownBorrow, [&](const APInt &Borrow) {
+ APInt Sub = N1 - N2;
+ if (Borrow.getBoolValue())
+ --Sub;
+
+ Known.One &= Sub;
+ Known.Zero &= ~Sub;
+ });
+ });
+ });
+
+ KnownBits KnownComputed =
+ KnownBits::computeForSubBorrow(Known1, Known2, KnownBorrow);
+ EXPECT_EQ(Known, KnownComputed);
+ });
+ });
+ });
+}
+
TEST(KnownBitsTest, BinaryExhaustive) {
testBinaryOpExhaustive(
[](const KnownBits &Known1, const KnownBits &Known2) {
More information about the llvm-commits
mailing list