https://bugs.llvm.org/show_bug.cgi?id=41459
Bug ID: 41459
Summary: __builtin_constant_p() appears to return true for
non-constant argument
Product: clang
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
Building an s390 linux allmodconfig kernel produced an error message for a
misoptimized code path:
pblk-rb.i:...: error: invalid operand for inline asm constraint 'i'
I managed to reduce the test case and make it architecture independent, test
with "clang-9 pblk-rb.i -Wall -O2 -S":
typedef struct { long counter; } atomic64_t;
static inline void __atomic64_add_const(long val, long *ptr)
{
asm volatile (" agsi %[ptr],%[val]"
:[ptr] "+Q"(*ptr)
:[val] "i"(val)
:"cc", "memory");
}
void __atomic64_add_var(long val, long *ptr);
static inline void atomic64_add(long i, atomic64_t * v)
{
if (__builtin_constant_p(i) && (i > -129) && (i < 128))
__atomic64_add_const(i, &v->counter);
else
__atomic64_add_var(i, &v->counter);
}
void pblk_rb_read_to_bio(atomic64_t *pblk, int count)
{
if (count)
atomic64_add(count, pblk);
atomic64_add(count, pblk);
}
The __builtin_constant_p() should never hit here, since 'count' is clearly not
constant. If a special case is made for 'count=0', then I would expect 'val' to
be 0 in that branch and fit into an "i" constraint.
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