avoid expensive assignement

include/xrpl/protocol/STAmount.h

@@ -45,6 +45,9 @@ public:
 
     static int const cMinOffset = -96;
     static int const cMaxOffset = 80;
+    // The -100 is used to allow 0 to sort less than small positive values
+    // which will have a large negative exponent.
+    static int const cZeroOffset = -100;
 
     // Maximum native value supported by the code
     constexpr static std::uint64_t cMinValue = 1'000'000'000'000'000ull;
@@ -524,7 +527,11 @@ STAmount::fromNumber(A const& a, Number const& number)
 
     auto const [mantissa, exponent] = working.normalizeToRange(cMinValue, cMaxValue);
 
-    return STAmount{asset, mantissa, exponent, negative};
+    // normalizeToRange produces values in canonical mantissa range [cMinValue, cMaxValue],
+    // but may produce out-of-range exponents for overflow/underflow cases.
+    // Use the regular constructor - canonicalize() will detect already-normalized mantissa
+    // and skip redundant scaling loops, while still handling overflow/underflow.
+    return STAmount{asset, static_cast<std::uint64_t>(mantissa), exponent, negative};
 }
 
 inline void
@@ -537,9 +544,7 @@ STAmount::negate()
 inline void
 STAmount::clear()
 {
-    // The -100 is used to allow 0 to sort less than a small positive values
-    // which have a negative exponent.
-    mOffset = integral() ? 0 : -100;
+    mOffset = integral() ? 0 : cZeroOffset;
     mValue = 0;
     mIsNegative = false;
 }

src/libxrpl/basics/Number.cpp

@@ -38,7 +38,10 @@ Number::getround()
 Number::rounding_mode
 Number::setround(rounding_mode mode)
 {
-    return std::exchange(mode_, mode);
+    auto const old = mode_;
+    if (old != mode)
+        mode_ = mode;
+    return old;
 }
 
 MantissaRange::mantissa_scale
@@ -52,6 +55,8 @@ Number::setMantissaScale(MantissaRange::mantissa_scale scale)
 {
     if (scale != MantissaRange::small && scale != MantissaRange::large)
         LogicError("Unknown mantissa scale");
+    if (range_.get().scale == scale)
+        return;
     range_ = scale == MantissaRange::small ? smallRange : largeRange;
 }
 

src/libxrpl/protocol/STAmount.cpp

@@ -866,34 +866,44 @@ STAmount::canonicalize()
 
     if (mValue == 0)
     {
-        mOffset = -100;
+        mOffset = cZeroOffset;
         mIsNegative = false;
         return;
     }
 
-    while ((mValue < cMinValue) && (mOffset > cMinOffset))
+    // Fast path: if mantissa is already in canonical range, skip scaling loops.
+    // This handles values from normalizeToRange that only need overflow/underflow checks.
+    bool const mantissaCanonical = (mValue >= cMinValue) && (mValue <= cMaxValue);
+
+    if (!mantissaCanonical)
     {
-        mValue *= 10;
-        --mOffset;
-    }
-
-    while (mValue > cMaxValue)
-    {
-        if (mOffset >= cMaxOffset)
-            Throw<std::runtime_error>("value overflow");
-
-        mValue /= 10;
-        ++mOffset;
+        // Mantissa needs normalization
+        while ((mValue < cMinValue) && (mOffset > cMinOffset))
+        {
+            mValue *= 10;
+            --mOffset;
+        }
+
+        while (mValue > cMaxValue)
+        {
+            if (mOffset >= cMaxOffset)
+                Throw<std::runtime_error>("value overflow");
+
+            mValue /= 10;
+            ++mOffset;
+        }
     }
 
+    // Check for underflow (applies whether we scaled or not)
     if ((mOffset < cMinOffset) || (mValue < cMinValue))
     {
         mValue = 0;
         mIsNegative = false;
-        mOffset = -100;
+        mOffset = cZeroOffset;
         return;
     }
 
+    // Check for overflow (applies whether we scaled or not)
     if (mOffset > cMaxOffset)
         Throw<std::runtime_error>("value overflow");
 
@@ -903,7 +913,7 @@ STAmount::canonicalize()
     XRPL_ASSERT(
         (mValue == 0) || ((mOffset >= cMinOffset) && (mOffset <= cMaxOffset)),
         "xrpl::STAmount::canonicalize : offset inside range");
-    XRPL_ASSERT((mValue != 0) || (mOffset != -100), "xrpl::STAmount::canonicalize : value or offset set");
+    XRPL_ASSERT((mValue != 0) || (mOffset == cZeroOffset), "xrpl::STAmount::canonicalize : value or offset set");
 }
 
 void

src/test/protocol/STAmount_test.cpp

@@ -1,7 +1,9 @@
 #include <test/jtx.h>
 
+#include <xrpl/basics/Number.h>
 #include <xrpl/basics/random.h>
 #include <xrpl/beast/unit_test.h>
+#include <xrpl/protocol/IOUAmount.h>
 #include <xrpl/protocol/STAmount.h>
 #include <xrpl/protocol/XRPAmount.h>
 
@@ -1143,6 +1145,148 @@ public:
         }
     }
 
+    void
+    testNumberConversion()
+    {
+        testcase("Number to STAmount conversions");
+
+        Issue const usd{Currency(0x5553440000000000), AccountID(0x4985601)};
+        NumberSO stNumberSO{true};
+
+        // Test zero conversion
+        {
+            Number const zero{};
+            STAmount const result{usd, zero};
+            BEAST_EXPECT(result.mantissa() == 0);
+            BEAST_EXPECT(result.exponent() == STAmount::cZeroOffset);
+            BEAST_EXPECT(!result.negative());
+        }
+
+        // Test positive zero
+        {
+            Number const zero{0, 0};
+            STAmount const result{usd, zero};
+            BEAST_EXPECT(result.mantissa() == 0);
+            BEAST_EXPECT(result.exponent() == STAmount::cZeroOffset);
+        }
+
+        // Test negative zero (should become positive zero)
+        {
+            Number const negZero{-0, 0};
+            STAmount const result{usd, negZero};
+            BEAST_EXPECT(result.mantissa() == 0);
+            BEAST_EXPECT(!result.negative());
+        }
+
+        // Test minimum positive IOU amount
+        {
+            Number const minPos{STAmount::cMinValue, STAmount::cMinOffset};
+            STAmount const result{usd, minPos};
+            BEAST_EXPECT(result.mantissa() == STAmount::cMinValue);
+            BEAST_EXPECT(result.exponent() == STAmount::cMinOffset);
+            BEAST_EXPECT(!result.negative());
+        }
+
+        // Test maximum positive IOU amount
+        {
+            Number const maxPos{STAmount::cMaxValue, STAmount::cMaxOffset};
+            STAmount const result{usd, maxPos};
+            BEAST_EXPECT(result.mantissa() == STAmount::cMaxValue);
+            BEAST_EXPECT(result.exponent() == STAmount::cMaxOffset);
+            BEAST_EXPECT(!result.negative());
+        }
+
+        // Test negative amounts
+        {
+            Number const neg{-static_cast<std::int64_t>(STAmount::cMinValue), STAmount::cMinOffset};
+            STAmount const result{usd, neg};
+            BEAST_EXPECT(result.mantissa() == STAmount::cMinValue);
+            BEAST_EXPECT(result.exponent() == STAmount::cMinOffset);
+            BEAST_EXPECT(result.negative());
+        }
+
+        // Test value requiring scale up (mantissa too small)
+        {
+            Number const small{1000000000000000ull / 10, -95};  // Will scale up
+            STAmount const result{usd, small};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+            BEAST_EXPECT(result.exponent() >= STAmount::cMinOffset);
+            BEAST_EXPECT(result.exponent() <= STAmount::cMaxOffset);
+        }
+
+        // Test value requiring scale down (mantissa too large)
+        {
+            Number const large{9999999999999999ull * 10, 79};  // Will scale down
+            STAmount const result{usd, large};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+            BEAST_EXPECT(result.exponent() >= STAmount::cMinOffset);
+            BEAST_EXPECT(result.exponent() <= STAmount::cMaxOffset);
+        }
+
+        // Test boundary mantissa values
+        {
+            Number const atMin{STAmount::cMinValue, 0};
+            STAmount const result{usd, atMin};
+            BEAST_EXPECT(result.mantissa() == STAmount::cMinValue);
+        }
+
+        {
+            Number const atMax{STAmount::cMaxValue, 0};
+            STAmount const result{usd, atMax};
+            BEAST_EXPECT(result.mantissa() == STAmount::cMaxValue);
+        }
+
+        // Test typical amounts
+        {
+            Number const typical{1234567890123456ull, -10};
+            STAmount const result{usd, typical};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+            BEAST_EXPECT(result.exponent() >= STAmount::cMinOffset);
+            BEAST_EXPECT(result.exponent() <= STAmount::cMaxOffset);
+        }
+
+        // Test round-trip conversion (Number -> STAmount -> Number)
+        {
+            Number const original{5000000000000000ull, 5};
+            STAmount const st{usd, original};
+            Number const recovered{st};
+            BEAST_EXPECT(original == recovered);
+        }
+
+        // Test various exponents
+        for (int exp = STAmount::cMinOffset; exp <= STAmount::cMaxOffset; exp += 10)
+        {
+            Number const n{STAmount::cMinValue, exp};
+            STAmount const result{usd, n};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+            BEAST_EXPECT(result.exponent() >= STAmount::cMinOffset);
+            BEAST_EXPECT(result.exponent() <= STAmount::cMaxOffset);
+        }
+
+        // Test both mantissa scales (if applicable)
+        {
+            // Small mantissa scale test
+            NumberMantissaScaleGuard guard{MantissaRange::small};
+            Number const n{5000000000000000ull, 5};
+            STAmount const result{usd, n};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+        }
+
+        {
+            // Large mantissa scale test
+            NumberMantissaScaleGuard guard{MantissaRange::large};
+            Number const n{5000000000000000ull, 5};
+            STAmount const result{usd, n};
+            BEAST_EXPECT(result.mantissa() >= STAmount::cMinValue);
+            BEAST_EXPECT(result.mantissa() <= STAmount::cMaxValue);
+        }
+    }
+
     //--------------------------------------------------------------------------
 
     void
@@ -1157,6 +1301,7 @@ public:
         testParseJson();
         testConvertXRP();
         testConvertIOU();
+        testNumberConversion();
         testCanAddXRP();
         testCanAddIOU();
         testCanAddMPT();