[llvm-bugs] [Bug 38789] New: warnings for untaken branch of ?: expression, at file scope

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38789

            Bug ID: 38789
           Summary: warnings for untaken branch of ?: expression, at file
                    scope
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arnd at linaro.org
                CC: llvm-bugs at lists.llvm.org

I have encountered a couple of warnings in the Linux kernel for range checking
of shift counts. The common macro

#define DMA_BIT_MASK(n)        (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))

produces a warning at file scope for a '64' argument, but not inside of
a function. I tried using __builtin_choose_expr() to avoid that, which
made it worse: that version always warns, regardless of the scope (it
also does this with gcc).

The same thing happened with a less common macro, which I've simplifed
into 'COND_BIT()' in the test case below.

The behavior is the same for all clang versions I tried (3.8 through 8),
but my expectation was to see no warning at all.

$ clang-8 -Wall -O2  -c -xc -  << EOF
#define DMA_BIT_MASK(n)        (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
#define DMA_BIT_MASK_2(n) __builtin_choose_expr((n) == 64, ~0ULL,
(1ULL<<(n))-1)
#define COND_BIT(x) (((x) >= 0) ? (1ul << x) : 0ul)
#define COND_BIT_2(x) __builtin_choose_expr(x >= 0, 1ul << x, 0ul)

// warning: shift count >= width of type [-Wshift-count-overflow]
long long a = DMA_BIT_MASK(64);

// warning: shift count >= width of type [-Wshift-count-overflow]
long long b = DMA_BIT_MASK_2(64);

// warning: shift count is negative [-Wshift-count-negative]
// [gcc: no warning]
unsigned long c = COND_BIT(-1);

// warning: shift count is negative [-Wshift-count-negative]
unsigned long d = COND_BIT_2(-1);

int f(void)
{
    // no warning
    long long a = DMA_BIT_MASK(64);

    // warning: shift count >= width of type [-Wshift-count-overflow]
    long long b = DMA_BIT_MASK_2(64);

    // no warning
    unsigned long c = COND_BIT(-1);

    // warning: shift count is negative [-Wshift-count-negative]
    unsigned long d = COND_BIT_2(-1);

    // prevent unused variable wanings
    return a & b & c & d;
}
EOF

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