Strange clang unused-but-set-variable error with volatile variables

Clang Frontend
1

In Linux kernel development, sometimes we use operations with volatile variables to burn cpu cycles for testing purpose. For example,

$ cat test.c
void test() {
        volatile int j = 0;
        for (int i = 0; i < 100000000; i++)
                j++;
        return;
}
$ clang -g -Wall -Werror -S -emit-llvm test.c
$

The clang is built with latest llvm-project source. For the above example, looking at test.ll file, the IR looks correct with proper load/stores related to volatile variables.

But a slight variation of the above code caused a compilation error.

$ cat test.c
void test() {
        volatile int j = 0;
        for (int i = 0; i < 100000000; i++)
                j+=1;
        return;
}
$ clang -g -Wall -Werror -S -emit-llvm test.c
test.c:2:15: error: variable 'j' set but not used [-Werror,-Wunused-but-set-variable]
        volatile int j = 0;
                     ^
1 error generated.
$

Can somebody help explain why the second example (with j+=1) cuased the compiler warning?
Currently we assume volatile variable, if involved with some operations, will never be optimized away, is this assumption true?

2

The code will work the way you expect. The warning is just that the construct looks suspicious. Maybe we should disable the warning for volatile variables.

3

@efriedma-quic Sounds a good idea to disable the warning for volatile variables. I am wondering what caused the second code sample failed but not the first one.

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