Bug 28344 - FCallCompare (mono_decimal_compare) returns incorrect result on arm and s390x
Summary: FCallCompare (mono_decimal_compare) returns incorrect result on arm and s390x
Alias: None
Product: Runtime
Classification: Mono
Component: General ()
Version: unspecified
Hardware: Other Linux
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2015-03-23 17:59 UTC by Neale Ferguson
Modified: 2015-03-25 01:33 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Neale Ferguson 2015-03-23 17:59:28 UTC
The following program works on 3.10.1 but fails on trunk:

using System;
using System.Reflection;
using System.IO;
using System.Text;

public class Class1
	static void Main()
		decimal value = 100;
		if (value < (decimal) Int32.MinValue || value > (decimal) Int32.MaxValue)

I've narrowed it down to the use of FCallCompare (mono_decimal_compare) that returns an incorrect value on arm and s390x. The code in question is the comparison of signs:

--- a/mono/metadata/decimal-ms.c
+++ b/mono/metadata/decimal-ms.c
@@ -2242,9 +2242,9 @@ mono_decimal_compare (MonoDecimal *left, MonoDecimal *right)
-       // Signs are different.  Used signed byte compares
+       // Signs are different.  Use signed byte comparison
-       if ((char)left_sign > (char)right_sign)
+       if ((signed char)left_sign > (signed char)right_sign)
                return MONO_DECIMAL_CMP_GT;
        return MONO_DECIMAL_CMP_LT;

Note the comment said "Used signed byte compares". However, it assumes that (char) == (signed chars) which for s390x is not true (it defaults to unsigned). By explicitly using (signed char) casting, the problem is fixed.

I pushed this change under commit ca5bebb67ce72ab267bee11685dcb5db13885474.

The purpose of this report is to highlight that some architectures default to unsigned char instead of signed.
Comment 1 Zoltan Varga 2015-03-25 01:33:45 UTC