Recently I discovered a bug in PHP’s decimal to floating-point conversion routine, zend_strtod(): it went into an infinite loop trying to convert the decimal string 2.2250738585072011e-308 to floating-point. zend_strtod() is based on David Gay’s strtod() function in dtoa.c, as are the decimal to floating-point conversion routines of many other open source projects. So why hasn’t this bug affected these other projects?
zend_strtod() is based on a very old copy of dtoa.c. The current version of dtoa.c is immune to the 2.2250738585072011e-308 bug — and has been since 1997 by my reckoning. So while the ‘volatile’ keyword fixes the PHP problem, I think there’s a better solution: upgrade zend_strtod() to the latest dtoa.c.
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Recently I discovered that Java converts some very small decimal numbers to double-precision floating-point incorrectly. While investigating that bug, I stumbled upon something very strange: Java’s decimal to floating-point conversion routine, Double.parseDouble(), sometimes returns two different results for the same decimal string. The culprit appears to be just-in-time compilation of Double.parseDouble() into SSE instructions, which exposes an architecture-dependent bug in Java’s conversion algorithm — and another real-world example of a double rounding on underflow error. I’ll describe the problem, and take you through the detective work to find its cause.
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While verifying the fix to the Java 2.2250738585072012e-308 bug I found an OpenJDK testcase for verifying conversions of edge case subnormal double-precision numbers. I ran the testcase, expecting it to work — but it failed! I determined it fails because Java converts some subnormal numbers incorrectly.
(By the way, this bug exists in prior versions of Java — it has nothing to do with the fix.)
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Oracle has released a fix for security alert CVE-2010-4476 — the “Java Hangs on 2.2250738585072012e-308 bug.” The fix comes in the form of something called the FPUpdater Tool, which updates rt.jar. I tested it on my Windows XP system and it works.
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Java’s decimal to floating-point conversion routine, the doubleValue() method of its FloatingDecimal class, goes into an infinite loop when converting the decimal string 2.2250738585072012e-308 to double-precision binary floating-point. I took a closer look at the bug, by tracing the doubleValue() method in the Eclipse IDE for Java (thanks to Konstantin Preißer for helping me set that up). What I found was that our initial analysis of the bug was wrong; what actually happens is that doubleValue()’s correction loop oscillates between two values, 0x1p-1022 and 0x0.fffffffffffffp-1022.
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