[Bug c/19449] New: __builtin_constant_p cannot resolve to const when optimizing

[Bug c/19449] New: __builtin_constant_p cannot resolve to const when optimizing

[eplondke at gmail dot com]

Given the following code:

#define SHIFTLEFT(a,b) __builtin_choose_expr(__builtin_constant_p(b), \
      ((a) << (b)), \
      ((a) << (MIN(31,MAX(-31,(b))))))

int foo(int a, int b) { return SHIFTLEFT (a, b); }

the compiler will error:
gcc -O4 gcctest.c 
gcctest.c: In function `foo':
gcctest.c:5: error: first argument to __builtin_choose_expr not a constant

According to the documentation to __builtin_constant_p, it should resolve
to 0 if the compiler cannot prove that the argument is a constant.

-- 
           Summary: __builtin_constant_p cannot resolve to const when
                    optimizing
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: eplondke at gmail dot com
                CC: eplondke at gmail dot com,gcc-bugs at gcc dot gnu dot
                    org
 GCC build triplet: i686-pc-cygwin
  GCC host triplet: i686-pc-cygwin
GCC target triplet: arm-unknown-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449

[Bug c/19449] __builtin_constant_p cannot resolve to const when optimizing

[ian at airs dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ian at airs dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449

[Bug c/19449] __builtin_constant_p cannot resolve to const when optimizing

[eplondke at gmail dot com]

------- Additional Comments From eplondke at gmail dot com  2005-01-14 19:22 -------
Created an attachment (id=7961)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=7961&action=view)
Test case as a file.

Test case as a file.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449

[Bug c/19449] __builtin_constant_p cannot resolve to const when optimizing

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-14 19:26 -------
The problem is the __builtin_constant_p is delayed for evalutation until optimizations are run but 
__builtin_choose_expr needs an answer right away.


What we could do is for when processing the first argument of __builtin_choose_expr, say we are not in 
a function.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
  GCC build triplet|i686-pc-cygwin              |
   GCC host triplet|i686-pc-cygwin              |
 GCC target triplet|arm-unknown-elf             |
           Keywords|                            |rejects-valid
   Last reconfirmed|0000-00-00 00:00:00         |2005-01-14 19:26:10
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449