[Bug c/28831] New: Aggregate copy not elided when using a return value as a pass-by-value parameter

[guillaume dot melquiond at ens-lyon dot fr]

Bug 23372 was a missed optimization with respect to GCC 3.4. It is now fixed
when the parameter is a reference. But there is still a regression when the
parameter is the return value of another function. Testcase: (-Wall -O3
--march=i386)

struct A { int a[1000]; };
struct A f();
void g(struct A);
void h() { g(f()); }

GCC 3.3 and 3.4 first allocate the stack frame of g and then require f to
directly store its return value in the parameter location. GCC 4.0, 4.1, and
4.2 (as of 2006-08-23) use another stack location for the return value of f,
then allocate the stack frame of g, and finally copy the value to this new
frame (possibly using a byte-by-byte copy, see bug 27055). The code generated
by GCC 3.x is optimal, the one by GCC 4.x is not.

GCC 3.4:
        movl    %esp, %eax
        subl    $12, %esp
        pushl   %eax
        call    f
        addl    $12, %esp
        call    g

GCC 4.2:
        leal    -4004(%ebp), %ebx
        pushl   %ebx
        call    f
        subl    $3988, %esp
        movl    %esp, %eax
        pushl   %edx
        pushl   $4000
        pushl   %ebx
        pushl   %eax
        call    memcpy
        addl    $16, %esp
        call    g

$ LANG=C /opt/gcc/bin/gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c,c++ --prefix=/opt/gcc
Thread model: posix
gcc version 4.2.0 20060823 (experimental)


-- 
           Summary: Aggregate copy not elided when using a return value as a
                    pass-by-value parameter
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr
GCC target triplet: i386-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831