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POCatching overflow of left shift of constant 1 using compiler warning?
 

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  1. POCatching overflow of left shift of constant 1 using compiler warning?
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    <p>We're writing code inside the Linux kernel so, try as I might, I wasn't able to get PC-Lint/Flexelint working on Linux kernel code. Just too many built-in symbols etc. But that's a side issue. </p> <p>We have any number of compilers, starting with gcc, but others also. Their warnings options have been getting stronger over time, to where they are pretty strong static analysis tools too.</p> <p>Here is what I want to catch. Yes, I know it violates some things that are easy to catch in code review, such as "no magic numbers", and "beware of bit shifting", but that's only if you happen to look at that section of code. Anyway, here it is:</p> <pre><code>unsigned long long foo; unsigned long bar; [... lots of other code ...] foo = ~(foo + (1&lt;&lt;bar)); </code></pre> <p>Further UPDATED problem description -- even with bar limited to 16, still a problem. Clarifying, the problem is implicit int type of constant that, unplanned, makes the complex expression violate the rule that all calculations be carried out in the same size and signedness.</p> <p>Problem: '1' is not long long, but, as a small-value constant, defaults to an int. Therefore even if bar's actual value never exceeds, say, 16, still the <code>(1&lt;&lt;bar)</code> expression will overflow and ruin the entire calculation.</p> <p>Possibly correct solution: write 1ULL instead.</p> <p>Is there a well-known compiler and compiler warning flag that will point out this (revised) problem?</p>
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      1. POCatching overflow of left shift of constant 1 using compiler warning?
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      1. VTUpMod
    1. CONo matter what size int is, if bar is too big then the shift is undefined (I think. Some kind of bad, anyway). You seem to be asking for the compiler to warn you about a variable second operand to shift on an int. But you say the problem is that '1' is not long: do you want no warning about a variable shift on a long? I don't see why compiler-writers would expect that warning to be useful, and general warnings about variable second operand to << would result in vast numbers of false positives.
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      1. POCatching overflow of left shift of constant 1 using compiler warning?
      1. USSteve Jessop
    2. CO@onebyone: Assuming `int` is 32 bits and `long long` is 64, `~(foo + (1<<bar))` only works for `0<=bar<32`, while `~(foo + (1LL<<bar))` works for `0<=bar<64`. I assume that OP wants a warning when the former is written instead of the latter.
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      1. POCatching overflow of left shift of constant 1 using compiler warning?
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    3. COSure, but would he want that warning in cases where foo is an int and bar is logically constrained (but not by type) to be either 1 or 2? How about where foo is a long long and bar is 1 or 2? Warnings for multiplying or adding bar to any signed constant, since the result of overflow is undefined? The compiler usually can't identify the logical constraints, so I'd expect any such warnings to be highly prone to false positives. Doesn't mean it's a bad idea, I'm just speculating why it's hard to define what it should be, and low priority for compiler writers anyway.
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      1. USSteve Jessop
 

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