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[APInt] Introduce carry-less multiply primitives #168527

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Merged
merged 3 commits into from
Nov 18, 2025
Merged

[APInt] Introduce carry-less multiply primitives #168527

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0fd708d
[APInt] Introduce carry-less multiply operation
artagnon Nov 18, 2025
a1ee9e0
[APInt] Avoid a shift with a branch
artagnon Nov 18, 2025
338998c
[APInt] Also introduce clmul[rh]
artagnon Nov 18, 2025
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21 changes: 21 additions & 0 deletions llvm/include/llvm/ADT/APInt.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2440,6 +2440,27 @@ LLVM_ABI APInt fshl(const APInt &Hi, const APInt &Lo, const APInt &Shift);
/// (4) fshr(i8 255, i8 0, i8 9) = fshr(i8 255, i8 0, i8 1) // 9 % 8
LLVM_ABI APInt fshr(const APInt &Hi, const APInt &Lo, const APInt &Shift);

/// Perform a carry-less multiply, also known as XOR multiplication, and return
/// low-bits. All arguments and result have the same bitwidth.
///
/// Examples:
/// (1) clmul(i4 1, i4 2) = 2
/// (2) clmul(i4 5, i4 6) = 14
/// (3) clmul(i4 -4, i4 2) = -8
/// (4) clmul(i4 -4, i4 -5) = 4
LLVM_ABI APInt clmul(const APInt &LHS, const APInt &RHS);

/// Perform a reversed carry-less multiply.
///
/// clmulr(a, b) = bitreverse(clmul(bitreverse(a), bitreverse(b)))
LLVM_ABI APInt clmulr(const APInt &LHS, const APInt &RHS);

/// Perform a carry-less multiply, and return high-bits. All arguments and
/// result have the same bitwidth.
///
/// clmulh(a, b) = clmulr(a, b) >> 1
LLVM_ABI APInt clmulh(const APInt &LHS, const APInt &RHS);

} // namespace APIntOps

// See friend declaration above. This additional declaration is required in
Expand Down
22 changes: 21 additions & 1 deletion llvm/lib/Support/APInt.cpp
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Expand Up @@ -15,10 +15,10 @@
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/FoldingSet.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/bit.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
Expand Down Expand Up @@ -3187,3 +3187,23 @@ APInt llvm::APIntOps::fshr(const APInt &Hi, const APInt &Lo,
return Lo;
return Hi.shl(Hi.getBitWidth() - ShiftAmt) | Lo.lshr(ShiftAmt);
}

APInt llvm::APIntOps::clmul(const APInt &LHS, const APInt &RHS) {
assert(LHS.getBitWidth() == RHS.getBitWidth());
unsigned BW = LHS.getBitWidth();
APInt Result(BW, 0);
for (unsigned I : seq<unsigned>(BW))
if (RHS[I])
Result ^= LHS.shl(I);
return Result;
}

APInt llvm::APIntOps::clmulr(const APInt &LHS, const APInt &RHS) {
assert(LHS.getBitWidth() == RHS.getBitWidth());
return clmul(LHS.reverseBits(), RHS.reverseBits()).reverseBits();
}

APInt llvm::APIntOps::clmulh(const APInt &LHS, const APInt &RHS) {
assert(LHS.getBitWidth() == RHS.getBitWidth());
return clmulr(LHS, RHS).lshr(1);
}
83 changes: 83 additions & 0 deletions llvm/unittests/ADT/APIntTest.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3823,4 +3823,87 @@ TEST(APIntTest, Fshr) {
-8193);
}

TEST(APIntTest, clmul) {
EXPECT_EQ(APIntOps::clmul(APInt(4, 1), APInt(4, 2)).getZExtValue(), 2U);
EXPECT_EQ(APIntOps::clmul(APInt(4, 5), APInt(4, 6)).getZExtValue(), 14U);
EXPECT_EQ(APIntOps::clmul(APInt(4, -4, /*isSigned*/ true),
APInt(4, 2, /*isSigned*/ false))
.getSExtValue(),
-8);
EXPECT_EQ(APIntOps::clmul(APInt(4, -4, /*isSigned*/ true),
APInt(4, -5, /*isSigned*/ true))
.getSExtValue(),
4);
EXPECT_EQ(APIntOps::clmul(APInt(8, 0), APInt(8, 255)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmul(APInt(8, 15), APInt(8, 15)).getZExtValue(), 85U);
EXPECT_EQ(APIntOps::clmul(APInt(8, 1), APInt(8, 2)).getZExtValue(), 2U);
EXPECT_EQ(APIntOps::clmul(APInt(64, 0, /*isSigned*/ true),
APInt(64, 9223372036854775807, /*isSigned*/ true))
.getSExtValue(),
0);
EXPECT_EQ(APIntOps::clmul(APInt(64, 1, /*isSigned*/ true),
APInt(64, 2, /*isSigned*/ true))
.getSExtValue(),
2);
EXPECT_EQ(APIntOps::clmul(APInt(16, -2, /*isSigned*/ true),
APInt(16, -1, /*isSigned*/ true))
.getSExtValue(),
-21846);
}

TEST(APIntTest, clmulr) {
EXPECT_EQ(APIntOps::clmulr(APInt(4, 1), APInt(4, 2)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulr(APInt(4, 5), APInt(4, 6)).getZExtValue(), 3U);
EXPECT_EQ(APIntOps::clmulr(APInt(4, -4, /*isSigned*/ true),
APInt(4, 2, /*isSigned*/ false))
.getSExtValue(),
3);
EXPECT_EQ(APIntOps::clmulr(APInt(4, -4, /*isSigned*/ true),
APInt(4, -5, /*isSigned*/ true))
.getSExtValue(),
-2);
EXPECT_EQ(APIntOps::clmulr(APInt(8, 0), APInt(8, 255)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulr(APInt(8, 15), APInt(8, 15)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulr(APInt(8, 1), APInt(8, 2)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulr(APInt(64, 0, /*isSigned*/ true),
APInt(64, 9223372036854775807, /*isSigned*/ true))
.getSExtValue(),
0);
EXPECT_EQ(APIntOps::clmulr(APInt(64, 1, /*isSigned*/ true),
APInt(64, 2, /*isSigned*/ true))
.getSExtValue(),
0);
EXPECT_EQ(APIntOps::clmulr(APInt(16, -2, /*isSigned*/ true),
APInt(16, -1, /*isSigned*/ true))
.getSExtValue(),
-21845);
}

TEST(APIntTest, clmulh) {
EXPECT_EQ(APIntOps::clmulh(APInt(4, 1), APInt(4, 2)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulh(APInt(4, 5), APInt(4, 6)).getZExtValue(), 1U);
EXPECT_EQ(APIntOps::clmulh(APInt(4, -4, /*isSigned*/ true),
APInt(4, 2, /*isSigned*/ false))
.getSExtValue(),
1);
EXPECT_EQ(APIntOps::clmulh(APInt(4, -4, /*isSigned*/ true),
APInt(4, -5, /*isSigned*/ true))
.getSExtValue(),
7);
EXPECT_EQ(APIntOps::clmulh(APInt(8, 0), APInt(8, 255)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulh(APInt(8, 15), APInt(8, 15)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulh(APInt(8, 1), APInt(8, 2)).getZExtValue(), 0U);
EXPECT_EQ(APIntOps::clmulh(APInt(64, 0, /*isSigned*/ true),
APInt(64, 9223372036854775807, /*isSigned*/ true))
.getSExtValue(),
0);
EXPECT_EQ(APIntOps::clmulh(APInt(64, 1, /*isSigned*/ true),
APInt(64, 2, /*isSigned*/ true))
.getSExtValue(),
0);
EXPECT_EQ(APIntOps::clmulh(APInt(16, -2, /*isSigned*/ true),
APInt(16, -1, /*isSigned*/ true))
.getSExtValue(),
21845);
}
} // end anonymous namespace
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