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Optimizing gcc to use lr / r14 in leaf functions...?

Nick Pelling
Note: This was originally posted on 5th November 2009 at http://forums.arm.com

Hi everyone,

I'm having difficulty convincing gcc to use r14 when cross-compiling C code for my most critical leaf function - arguably the oldest ARM hand-coding trick in the book (yes, I had an Archimedes), yet it's just not happening here. :-(

Is there some kind of secret gcc option-fu trickery that enables this? Or are there any special conditions which make gcc think that non-function-calling functions should not be treated as leaf functions?

Any suggestions or comments much appreciated!

Cheers, ....Nick Pelling....

PS: this is what "arm-elf-gcc --v" returns, in case this is a silent side-effect of one of the preconfigured options:-

Configured with: ../gcc-4.4.1/configure --target=arm-elf --prefix=/home/yagarto/install --disable-nls --disable-shared --disable-threads --with-gcc --with-gnu-ld --with-gnu-as --with-dwarf2 --enable-languages=c,c++ --enable-interwork --enable-multilib --with-newlib --with-headers=../newlib-1.17.0/newlib/libc/include --disable-libssp --disable-libstdcxx-pch --disable-libmudflap --disable-libgomp -v

Thread model: single
gcc version 4.4.1 (GCC)


...and this is the basic command line I'm using (note that I've tried both -O2 and -O3 optimization levels without success)...

arm-elf-gcc -mtune=arm926ej-s -Wall -g -O2 -c -o obj\main.o main.c


PPS: might it conceivably be that the arm926ej-s does not allow r14 to be used as a completely general-purpose register?
Parents
  • Arm Employee Badge
    Note: This was originally posted on 6th November 2009 at http://forums.arm.com

    Are you sure it's not just a feature of the code you're giving it? The following [admittedly contrived] code will use LR as one of the loop iterators when compiled with gcc4.2.1, but only with -O3 :

    int foo(int r0) {
    int r1,r2,r3,r4,r5,r6,r7,r8,r9,r10,r11,r12,r13,r14;

    r1 = 0;

    for(r2=0;r2<r0;r2++)
    for(r3=0;r3<r0;r3++)
    for(r4=0;r4<r0;r4++)
    for(r5=0;r5<r0;r5++)
    for(r6=0;r6<r0;r6++)
    for(r7=0;r7<r0;r7++)
    for(r8=0;r8<r0;r8++)
    for(r9=0;r9<r0;r9++)
    for(r10=0;r10<r0;r10++)
    for(r11=0;r11<r0;r11++)
    for(r12=0;r12<r0;r12++)
    for(r13=0;r13<r0;r13++)
    for(r14=0;r14<r0;r14++)
      r1++;

    return r1;
    }

    hth
    s.
Reply
  • Arm Employee Badge
    Note: This was originally posted on 6th November 2009 at http://forums.arm.com

    Are you sure it's not just a feature of the code you're giving it? The following [admittedly contrived] code will use LR as one of the loop iterators when compiled with gcc4.2.1, but only with -O3 :

    int foo(int r0) {
    int r1,r2,r3,r4,r5,r6,r7,r8,r9,r10,r11,r12,r13,r14;

    r1 = 0;

    for(r2=0;r2<r0;r2++)
    for(r3=0;r3<r0;r3++)
    for(r4=0;r4<r0;r4++)
    for(r5=0;r5<r0;r5++)
    for(r6=0;r6<r0;r6++)
    for(r7=0;r7<r0;r7++)
    for(r8=0;r8<r0;r8++)
    for(r9=0;r9<r0;r9++)
    for(r10=0;r10<r0;r10++)
    for(r11=0;r11<r0;r11++)
    for(r12=0;r12<r0;r12++)
    for(r13=0;r13<r0;r13++)
    for(r14=0;r14<r0;r14++)
      r1++;

    return r1;
    }

    hth
    s.
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