[Bug tree-optimization/28778] [4.0/4.1 Regression] alias bug with cast and call clobbered

["chuck at vertica dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #46 from chuck at vertica dot com  2006-11-05 22:17 -------
Folks, can anyone please tell me if this is the same problem as I am seeing
here using gcc 4.0.2 for x86_64:

#include <stdio.h>

inline long long Vgetbytes(double f) {
   return *reinterpret_cast<const long long *>(&f);
}

int main (int argc, char **argv)
{
    double dd = 2.0;

    printf ("%llx\n", Vgetbytes(dd));
    printf ("%llx\n", Vgetbytes(dd));
}

When compiled with -02 I get:
build0:/home/cbear $ g++4 --version
g++4 (GCC) 4.0.2 20051130 (Red Hat 4.0.2-14.EL4)
build0:/home/cbear $ g++4 -O2 ftp.cpp
build0:/home/cbear $ ./a.out
0
4000000000000000

When I look at the disassemby I see the problem:
main:
.LFB14:
        pushq   %rbx    # Save EBX as required
.LCFI0:
        movl    $.LC1, %edi     #, Load format string for printf. edi = arg 0
        movabsq $4611686018427387904, %rbx      # rbx gets 2.0.
        xorl    %eax, %eax      # irrelevant
        subq    $16, %rsp       # Stack frame for calling printf
.LCFI1:
        movq    8(%rsp), %rsi   # ERROR second argument to printf call wrong
        movq    %rbx, 8(%rsp)   # ERROR if this line came before prior we would
be OK
        call    printf          # printf gets rdi and rsi as args.  rsi bad.
        movq    8(%rsp), %rsi   # Better luck this time, rsi set properly
        movl    $.LC1, %edi     # rdi initialized againg
        xorl    %eax, %eax      # irrelevant
        movq    %rbx, 8(%rsp)   # extraneous, rbx need not be preserved
        call    printf          # This one will work
        addq    $16, %rsp       # Clean up the printf stack
        xorl    %eax, %eax      # <result>
        popq    %rbx            # restore rbx for caller
        ret                     # done

If I compile with -fno-strict-aliasing I do get valid code and right answers.

However I like the assembly code produced by gcc 3.4.4 much better because it
doesn't bother to allocate stack space, and just uses rbx:
build0:/home/cbear $ g++ --version
g++ (GCC) 3.4.4 20050721 (Red Hat 3.4.4-2)
build0:/home/cbear $ g++ -O2 -S -fverbose-asm ftp.cpp

main:
.LFB14:
        pushq   %rbx    #
.LCFI0:
        movabsq $4611686018427387904, %rbx      #, <anonymous>
        movl    $.LC1, %edi     #,
        movq    %rbx, %rsi      # <anonymous>, <anonymous>
        xorl    %eax, %eax      #
        call    printf  #
        movq    %rbx, %rsi      # <anonymous>, <anonymous>
        movl    $.LC1, %edi     #,
        xorl    %eax, %eax      #
        call    printf  #
        popq    %rbx    #
        xorl    %eax, %eax      # <result>
        ret


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28778