From: "mrkn (Kenta Murata)" <muraken@...>
Date: 2013-12-13T02:18:35+09:00
Subject: [ruby-core:59076] [ruby-trunk - Bug #7371][Closed] Fix undefined	overflow checking in bigdecimal


Issue #7371 has been updated by mrkn (Kenta Murata).

Status changed from Assigned to Closed

I think this issue had been fixed by akr in r40214.
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Bug #7371: Fix undefined overflow checking in bigdecimal
https://bugs.ruby-lang.org/issues/7371#change-43629

Author: xi (Xi Wang)
Status: Closed
Priority: Normal
Assignee: mrkn (Kenta Murata)
Category: ext
Target version: 
ruby -v: 1.9.x
Backport: 


In AddExponent() at ext/bigdecimal/bigdecimal.c:3677, the overflow checks rely on signed integer overflow, which is undefined behavior in C.

    SIGNED_VALUE m = e+n;
    SIGNED_VALUE eb, mb;
    if(e>0) {
        if(n>0) {
            mb = m*(SIGNED_VALUE)BASE_FIG;
            eb = e*(SIGNED_VALUE)BASE_FIG;
            if(mb<eb) goto overflow;
        }

Some compilers (e.g., gcc 4.8) will optimize away such overflow checks due to undefined behavior.  Ruby currently uses "-fno-strict-overflow" to disable such offending optimizations in gcc, but this workaround option is not supported by other compilers, thus not portable.

The attached patch uses unsigned multiplication for overflow checking, which is well defined in C.


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