[llvm] 0a07ae6 - [KnownBits] Add support for X*X self-multiplication
Simon Pilgrim via llvm-commits
llvm-commits at lists.llvm.org
Tue Sep 7 04:00:57 PDT 2021
Author: Simon Pilgrim
Date: 2021-09-07T11:43:45+01:00
New Revision: 0a07ae6ebfffb77243787f941d02f281a0041762
URL: https://github.com/llvm/llvm-project/commit/0a07ae6ebfffb77243787f941d02f281a0041762
DIFF: https://github.com/llvm/llvm-project/commit/0a07ae6ebfffb77243787f941d02f281a0041762.diff
LOG: [KnownBits] Add support for X*X self-multiplication
Add KnownBits handling and unit tests for X*X self-multiplication cases which guarantee that bit1 of their results will be zero - see PR48683.
https://alive2.llvm.org/ce/z/NN_eaR
The next step will be to add suitable test coverage so this can be enabled in ValueTracking/DAG/GlobalISel - currently only a single Analysis/ScalarEvolution test is affected.
Differential Revision: https://reviews.llvm.org/D108992
Added:
Modified:
llvm/include/llvm/Support/KnownBits.h
llvm/lib/Support/KnownBits.cpp
llvm/unittests/Support/KnownBitsTest.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index cfec5796493f..cd99575242e6 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -304,7 +304,8 @@ struct KnownBits {
KnownBits RHS);
/// Compute known bits resulting from multiplying LHS and RHS.
- static KnownBits mul(const KnownBits &LHS, const KnownBits &RHS);
+ static KnownBits mul(const KnownBits &LHS, const KnownBits &RHS,
+ bool SelfMultiply = false);
/// Compute known bits from sign-extended multiply-hi.
static KnownBits mulhs(const KnownBits &LHS, const KnownBits &RHS);
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index d997bd85f1e0..5f7281b91f93 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -412,10 +412,13 @@ KnownBits KnownBits::abs(bool IntMinIsPoison) const {
return KnownAbs;
}
-KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS) {
+KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,
+ bool SelfMultiply) {
unsigned BitWidth = LHS.getBitWidth();
assert(BitWidth == RHS.getBitWidth() && !LHS.hasConflict() &&
!RHS.hasConflict() && "Operand mismatch");
+ assert((!SelfMultiply || (LHS.One == RHS.One && LHS.Zero == RHS.Zero)) &&
+ "Self multiplication knownbits mismatch");
// Compute a conservative estimate for high known-0 bits.
unsigned LeadZ =
@@ -489,6 +492,14 @@ KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS) {
Res.Zero.setHighBits(LeadZ);
Res.Zero |= (~BottomKnown).getLoBits(ResultBitsKnown);
Res.One = BottomKnown.getLoBits(ResultBitsKnown);
+
+ // If we're self-multiplying then bit[1] is guaranteed to be zero.
+ if (SelfMultiply && BitWidth > 1) {
+ assert(Res.One[1] == 0 &&
+ "Self-multiplication failed Quadratic Reciprocity!");
+ Res.Zero.setBit(1);
+ }
+
return Res;
}
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
index cce426748a6f..6b5d1b56a94a 100644
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -267,6 +267,23 @@ TEST(KnownBitsTest, BinaryExhaustive) {
EXPECT_TRUE(ComputedAShr.One.isSubsetOf(KnownAShr.One));
});
});
+
+ // Also test 'unary' binary cases where the same argument is repeated.
+ ForeachKnownBits(Bits, [&](const KnownBits &Known) {
+ KnownBits KnownMul(Bits);
+ KnownMul.Zero.setAllBits();
+ KnownMul.One.setAllBits();
+
+ ForeachNumInKnownBits(Known, [&](const APInt &N) {
+ APInt Res = N * N;
+ KnownMul.One &= Res;
+ KnownMul.Zero &= ~Res;
+ });
+
+ KnownBits ComputedMul = KnownBits::mul(Known, Known, /*SelfMultiply*/ true);
+ EXPECT_TRUE(ComputedMul.Zero.isSubsetOf(KnownMul.Zero));
+ EXPECT_TRUE(ComputedMul.One.isSubsetOf(KnownMul.One));
+ });
}
TEST(KnownBitsTest, UnaryExhaustive) {
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