[Bug optimization/14121] New: use inline assembler register constraints as home register hints

["thomas dot schlichter at web dot de" <gcc-bugzilla@gcc.gnu.org>]

Hi,

gcc 3.4.0 (and 3.3.1) seems not to use the register constraints of the inline
assembler as hints where to place local variables. For example following part of
the file bin2dec.c (will provide it later) :

    int m2 = m;
    unsigned long c = 0;

    do {
      asm("divl\t%2" : "+a"(x[m2]), "+d"(c) : "r"(1000000000));
    } while (--m2 >= 0);

is translated to following x86 assembler instructions (gcc -O2 -S):

        movl    $1000000000, %ebx
        .p2align 4,,15
.L3:
        movl    %edi, -20(%ebp)
        xorl    %ecx, %ecx
        .p2align 4,,15
.L6:
        movl    -20(%ebp), %edx
        movl    (%esi,%edx,4), %eax
        movl    %ecx, %edx
#APP
        divl    %ebx
#NO_APP
        movl    %edx, %ecx
        movl    -20(%ebp), %edx
        movl    %eax, (%esi,%edx,4)
        decl    %edx
        movl    %edx, -20(%ebp)
        jns     .L6

So the variable "c" seems to be placed in the register "ecx" whereas it is
constrained to register "edx". If "c" was placed in "edx" the register "ecx"
would become free and could be used for "m2". So this variable would not have to
be placed on the stack...

gcc 2.95.3 seems to do this placement, but I don't know if it does this
coincidental or intentionally... It generates following code (ebx<>m, ecx<>m2,
edx<>c, edi<>x):

        movl %ebx,%ecx
        xorl %edx,%edx
        .p2align 2,0x90
.L10:
        movl (%edi,%ecx,4),%eax
        movl $1000000000,%esi
#APP
        divl    %esi
#NO_APP
        movl %eax,(%edi,%ecx,4)
        decl %ecx
        jns .L10

So it would be GREAT if 3.4.0 would do anything like that (plus moving the
assignment to esi outside the loop, of course ;-)

Best reagrds
   Thomas Schlichter

-- 
           Summary: use inline assembler register constraints as home
                    register hints
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: thomas dot schlichter at web dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14121