[llvm] 0b293e8 - [APInt] Remove multiplicativeInverse with explicit modulus (#87644)

[via llvm-commits]

Author: Jay Foad
Date: 2024-04-04T17:24:16+01:00
New Revision: 0b293e8c36d97bbd7f85ed5b67ce510ff7fd86ee

URL: https://github.com/llvm/llvm-project/commit/0b293e8c36d97bbd7f85ed5b67ce510ff7fd86ee
DIFF: https://github.com/llvm/llvm-project/commit/0b293e8c36d97bbd7f85ed5b67ce510ff7fd86ee.diff

LOG: [APInt] Remove multiplicativeInverse with explicit modulus (#87644)

All callers have been changed to use the new simpler overload with an
implicit modulus of 2^BitWidth. The old form was never used or tested
with non-power-of-two modulus anyway.

Added: 
    

Modified: 
    llvm/include/llvm/ADT/APInt.h
    llvm/lib/Support/APInt.cpp
    llvm/unittests/ADT/APIntTest.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index bd1716219ee5fc..8d3c029b2e7e91 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -1740,9 +1740,6 @@ class [[nodiscard]] APInt {
     return *this;
   }
 
-  /// \returns the multiplicative inverse for a given modulo.
-  APInt multiplicativeInverse(const APInt &modulo) const;
-
   /// \returns the multiplicative inverse of an odd APInt modulo 2^BitWidth.
   APInt multiplicativeInverse() const;
 

diff  --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index f8f699f8f6ccd7..224ea0924f0aaa 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -1240,55 +1240,6 @@ APInt APInt::sqrt() const {
   return x_old + 1;
 }
 
-/// Computes the multiplicative inverse of this APInt for a given modulo. The
-/// iterative extended Euclidean algorithm is used to solve for this value,
-/// however we simplify it to speed up calculating only the inverse, and take
-/// advantage of div+rem calculations. We also use some tricks to avoid copying
-/// (potentially large) APInts around.
-/// WARNING: a value of '0' may be returned,
-///          signifying that no multiplicative inverse exists!
-APInt APInt::multiplicativeInverse(const APInt& modulo) const {
-  assert(ult(modulo) && "This APInt must be smaller than the modulo");
-
-  // Using the properties listed at the following web page (accessed 06/21/08):
-  //   http://www.numbertheory.org/php/euclid.html
-  // (especially the properties numbered 3, 4 and 9) it can be proved that
-  // BitWidth bits suffice for all the computations in the algorithm implemented
-  // below. More precisely, this number of bits suffice if the multiplicative
-  // inverse exists, but may not suffice for the general extended Euclidean
-  // algorithm.
-
-  APInt r[2] = { modulo, *this };
-  APInt t[2] = { APInt(BitWidth, 0), APInt(BitWidth, 1) };
-  APInt q(BitWidth, 0);
-
-  unsigned i;
-  for (i = 0; r[i^1] != 0; i ^= 1) {
-    // An overview of the math without the confusing bit-flipping:
-    // q = r[i-2] / r[i-1]
-    // r[i] = r[i-2] % r[i-1]
-    // t[i] = t[i-2] - t[i-1] * q
-    udivrem(r[i], r[i^1], q, r[i]);
-    t[i] -= t[i^1] * q;
-  }
-
-  // If this APInt and the modulo are not coprime, there is no multiplicative
-  // inverse, so return 0. We check this by looking at the next-to-last
-  // remainder, which is the gcd(*this,modulo) as calculated by the Euclidean
-  // algorithm.
-  if (r[i] != 1)
-    return APInt(BitWidth, 0);
-
-  // The next-to-last t is the multiplicative inverse.  However, we are
-  // interested in a positive inverse. Calculate a positive one from a negative
-  // one if necessary. A simple addition of the modulo suffices because
-  // abs(t[i]) is known to be less than *this/2 (see the link above).
-  if (t[i].isNegative())
-    t[i] += modulo;
-
-  return std::move(t[i]);
-}
-
 /// \returns the multiplicative inverse of an odd APInt modulo 2^BitWidth.
 APInt APInt::multiplicativeInverse() const {
   assert((*this)[0] &&

diff  --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 23f9ee2d39c441..76fc26412407e7 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -3249,22 +3249,11 @@ TEST(APIntTest, SolveQuadraticEquationWrap) {
 }
 
 TEST(APIntTest, MultiplicativeInverseExaustive) {
-  for (unsigned BitWidth = 1; BitWidth <= 16; ++BitWidth) {
-    for (unsigned Value = 0; Value < (1u << BitWidth); ++Value) {
+  for (unsigned BitWidth = 1; BitWidth <= 8; ++BitWidth) {
+    for (unsigned Value = 1; Value < (1u << BitWidth); Value += 2) {
+      // Multiplicative inverse exists for all odd numbers.
       APInt V = APInt(BitWidth, Value);
-      APInt MulInv =
-          V.zext(BitWidth + 1)
-              .multiplicativeInverse(APInt::getSignedMinValue(BitWidth + 1))
-              .trunc(BitWidth);
-      APInt One = V * MulInv;
-      if (V[0]) {
-        // Multiplicative inverse exists for all odd numbers.
-        EXPECT_TRUE(One.isOne());
-        EXPECT_TRUE((V * V.multiplicativeInverse()).isOne());
-      } else {
-        // Multiplicative inverse does not exist for even numbers (and 0).
-        EXPECT_TRUE(MulInv.isZero());
-      }
+      EXPECT_EQ(V * V.multiplicativeInverse(), 1);
     }
   }
 }