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

Category set to ext
Assignee set to mrkn (Kenta Murata)


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Bug #7371: Fix undefined overflow checking in bigdecimal
https://bugs.ruby-lang.org/issues/7371#change-33009

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


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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