[llvm] [Support][APInt] Fix sign extension, exponent and mantissa in APInt::roundToDouble (PR #192451)

[Andi Drebes via llvm-commits]

https://github.com/andidr updated https://github.com/llvm/llvm-project/pull/192451

>From 0b4c3f81ac98bd553eec3c27d075a1e86cfd1a3b Mon Sep 17 00:00:00 2001
From: Andi Drebes <andi at drebesium.org>
Date: Thu, 16 Apr 2026 15:05:32 +0200
Subject: [PATCH] [Support][APInt] Fix sign extension, exponent and mantissa in
 APInt::roundToDouble

Conversion of an `APInt` to double via `APInt::roundToDouble` misses
several edge cases that result in crashes due to an assertion or in
erroneous values due to incorrect values for the exponent and mantissa
of the generated double.

The assertion is triggered when attempting to convert a multi-word
`APInt` without active bits or with active bits only in the first
word, e.g.,

  `APInt(65, 0, true).roundToDouble(true)` or
  `APInt(65, 1, true).roundToDouble(true)`

The issue is caused by passing the bit width as the argument to
`SignExtend64`, exceeding the maximum expected bit width of 64.

Incorrect values for the mantissa or exponent are calculated for any
multi-word, unsigned value, e.g.,

  `APInt(65, "18446744073709551616" /* 2^64 */, 10).roundToDouble(false)`

This is due to `APInt::roundToDouble` expecting the exponent to
correspond to the highest of the fractional part of the double rather
than to the highest active bit and due to the missing code clearing
the highest bit after generation of the mantissa.

This patch solves the issues by simplifying the treatment of
single-word integers and by adjusting the exponent and mantissa for
multi-word integers. Tests are added for the edge cases and some
regular cases.
---
 llvm/lib/Support/APInt.cpp       | 23 +++++++++++---------
 llvm/unittests/ADT/APIntTest.cpp | 37 ++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 6aa5fc615a302..1f21c82b8a4f4 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -900,12 +900,9 @@ APInt llvm::APIntOps::RoundDoubleToAPInt(double Double, unsigned width) {
 double APInt::roundToDouble(bool isSigned) const {
   // Handle the simple case where the value is contained in one uint64_t.
   // It is wrong to optimize getWord(0) to VAL; there might be more than one word.
-  if (isSingleWord() || getActiveBits() <= APINT_BITS_PER_WORD) {
-    if (isSigned) {
-      int64_t sext = SignExtend64(getWord(0), BitWidth);
-      return double(sext);
-    }
-    return double(getWord(0));
+  if (isSingleWord()) {
+    return isSigned ? double(bit_cast<int64_t>(getWord(0)))
+                    : double(getWord(0));
   }
 
   // Determine if the value is negative.
@@ -917,10 +914,15 @@ double APInt::roundToDouble(bool isSigned) const {
   // Figure out how many bits we're using.
   unsigned n = Tmp.getActiveBits();
 
-  // The exponent (without bias normalization) is just the number of bits
-  // we are using. Note that the sign bit is gone since we constructed the
-  // absolute value.
-  uint64_t exp = n;
+  // Early exit for 0 to avoid negative indexes
+  if (n == 0)
+    return 0.0;
+
+  // The exponent (without bias normalization) is just the number of
+  // bits we are using (minus 1 to account for the fact that the
+  // exponent is on 2). Note that the sign bit is gone since we
+  // constructed the absolute value.
+  uint64_t exp = n - 1;
 
   // Return infinity for exponent overflow
   if (exp > 1023) {
@@ -947,6 +949,7 @@ double APInt::roundToDouble(bool isSigned) const {
   }
 
   // The leading bit of mantissa is implicit, so get rid of it.
+  mantissa &= ~(1ULL << std::min(n - 1, 51U));
   uint64_t sign = isNeg ? (1ULL << (APINT_BITS_PER_WORD - 1)) : 0;
   uint64_t I = sign | (exp << 52) | mantissa;
   return bit_cast<double>(I);
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index ee4c59de34fc4..dd5ee556f6306 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -3980,4 +3980,41 @@ TEST(APIntTest, clmulh) {
                 .getSExtValue(),
             21845);
 }
+
+TEST(APIntTest, roundToDouble) {
+  // Single-word, positive
+  EXPECT_EQ(APInt(64, 0, false).roundToDouble(false), 0.0);
+  EXPECT_EQ(APInt(64, 1, false).roundToDouble(false), 1.0);
+  EXPECT_EQ(APInt(64, 2, false).roundToDouble(false), 2.0);
+  EXPECT_EQ(APInt(64, 1ULL << 63, false).roundToDouble(false),
+            9223372036854775808.0);
+
+  // Single-word, negative
+  EXPECT_EQ(APInt(64, 0, true).roundToDouble(true), 0.0);
+  EXPECT_EQ(APInt(64, -1, true).roundToDouble(true), -1.0);
+  EXPECT_EQ(APInt(64, -2, true).roundToDouble(true), -2.0);
+  EXPECT_EQ(APInt(64, 1ULL << 63, true).roundToDouble(true),
+            -9223372036854775808.0);
+
+  // Multi-word, positive, active bits in first word
+  EXPECT_EQ(APInt(65, 0, false).roundToDouble(false), 0.0);
+  EXPECT_EQ(APInt(65, 1, false).roundToDouble(false), 1.0);
+  EXPECT_EQ(APInt(65, 2, false).roundToDouble(false), 2.0);
+  EXPECT_EQ(APInt(65, 1ULL << 63, false).roundToDouble(true),
+            9223372036854775808.0);
+
+  // Multi-word, positive, active bits outside first word
+  EXPECT_EQ(
+      APInt(65, "18446744073709551616" /* 2^64 */, 10).roundToDouble(false),
+      18446744073709551616.0);
+
+  // Multi-word, negative
+  EXPECT_EQ(APInt(65, 0, true).roundToDouble(true), 0.0);
+
+  EXPECT_EQ(APInt(65, -1, true).roundToDouble(true), -1.0);
+  EXPECT_EQ(APInt(65, -2, true).roundToDouble(true), -2.0);
+  EXPECT_EQ(
+      APInt(65, "18446744073709551616" /* 2^64 */, 10).roundToDouble(true),
+      -18446744073709551616.0);
+}
 } // end anonymous namespace