From: "xi (Xi Wang)" <redmine@...>
Date: 2012-12-31T14:01:43+09:00
Subject: [ruby-core:51205] [ruby-trunk - Bug #7371] Fix undefined overflow checking in bigdecimal


Issue #7371 has been updated by xi (Xi Wang).


To see how it works, try to compile the following (simplified) code with gcc 4.8.  The entire function will be optimized away with "gcc -O2" (just grep "bar" in the resulting assembly code); gcc 4.7 or earlier doesn't do that.

#define SIGNED_VALUE    long
#define BASE_FIG        9

void bar(void);

static void AddExponent(SIGNED_VALUE e, SIGNED_VALUE n)
{
    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;
        }
    }
    return;
overflow:
    bar();
}

void foo(SIGNED_VALUE e)
{
    AddExponent(e, 1);
}
----------------------------------------
Bug #7371: Fix undefined overflow checking in bigdecimal
https://bugs.ruby-lang.org/issues/7371#change-35165

Author: xi (Xi Wang)
Status: Assigned
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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