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


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
        movl    %edi, -20(%ebp)
        xorl    %ecx, %ecx
        .p2align 4,,15
        movl    -20(%ebp), %edx
        movl    (%esi,%edx,4), %eax
        movl    %ecx, %edx
        divl    %ebx
        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
        movl (%edi,%ecx,4),%eax
        movl $1000000000,%esi
        divl    %esi
        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

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