[LLVMbugs] [Bug 9355] New: missing DCE for certain constant expressions?

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9355

           Summary: missing DCE for certain constant expressions?
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: pageexec at freemail.hu
                CC: llvmbugs at cs.uiuc.edu


cloning comment 10 from bug 9322 per Ted's request:

example in linux/include/linux/bitops.h (code's compiled with -O2):

static inline unsigned long hweight_long(unsigned long w)
{
        return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
}

on i386 this should not evaluate hweight64 as that'll result in warnings like
this:

In file included from lib/halfmd4.c:1:
In file included from include/linux/kernel.h:17:
include/linux/bitops.h:49:41: warning: shift count >= width of type
        return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
                                               ^~~~~~~~~~~~
as the macro breaks down to this:

include/asm-generic/bitops/const_hweight.h:27:49: note: instantiated from:
#define hweight64(w) (__builtin_constant_p(w) ? __const_hweight64(w) :
__arch_hweight64(w))
                                                ^
include/asm-generic/bitops/const_hweight.h:19:54: note: instantiated from:
#define __const_hweight64(w) (__const_hweight32(w) + __const_hweight32((w) >>
32))

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