[cfe-dev] __builtin_assume removes calls to functions with side effect

[David Majnemer via cfe-dev]

Should be fixed in r273711.

On Fri, Jun 24, 2016 at 11:29 AM, David Majnemer <david.majnemer at gmail.com>
wrote:

> This looks like a bug to me...  It seems that callers of
> SimplifyInstruction are not checking to see if the replaced instruction is
> side-effect free.
>
> On Fri, Jun 24, 2016 at 10:37 AM, u3shit via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hello,
>>
>> [Please CC me when replying, I'm not on the list]
>>
>> I've been experimenting with replacing assert with __builtin_assume in
>> release builds, and I've hit a problem. I did not open a bug because I'm
>> not sure if this is really a bug, or just the documentation is not clear
>> enough. Anyway, look at this simple program:
>>
>> #include <stdio.h>
>>
>> int global;
>>
>> int foo()
>> {
>>     global = 1;
>>     return 3;
>> }
>>
>> int main()
>> {
>>     int x = foo();
>>     __builtin_assume(x == 3);
>>     printf("%d %d\n", x, global);
>>     return 0;
>> }
>>
>>
>> If I try to run it:
>> $ clang++ -O2 foo.cpp && ./a.out
>> 3 0
>>
>> (If I compile with -O0, I get the expected "3 1" as output). The
>> documentation itself says "The argument itself is never evaluated, so
>> any side effects of the expression will be discarded.", but the argument
>> here is "x == 3", not "foo() == 3", so IMHO it shouldn't eliminate the
>> function call here. Am I right here?
>>
>> I get the same result from clang 3.8.0 from gentoo and from the git
>> master.
>>
>> Thanks.
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>> cfe-dev at lists.llvm.org
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>>
>
>
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