llvm optimizes with library functions

Question

Starting with code like this

void lib_memset( unsigned char *dest, unsigned char c, unsigned int n)
{
    while(n--)
    {
        *dest=c;
        dest++;
          


        

Answer 1

clang -Wall -m32 -emit-llvm -fno-builtin --target=arm-none-eabi -ffreestanding -c lib_memset.c -o lib_memset.bc
opt -std-compile-opts -strip-debug -march=arm -mcpu=mpcore -mtriple=arm-none-eabi -disable-simplify-libcalls  lib_memset.bc -o lib_memset.opt.bc
llc -march=arm -mcpu=mpcore -disable-simplify-libcalls -mtriple=arm-none-eabi  lib_memset.opt.bc -o lib_memset.opt.s
llc -march=arm -mcpu=mpcore -disable-simplify-libcalls -mtriple=arm-none-eabi  lib_memset.bc -o lib_memset.s

thanks to artless noise while adding -ffreestanding, I decided to re-read all the --help options for llc and opt, and found the -disable-simpilfy-libcalls is an option for both opt and llc, adding it to opt fixed the problem.

lib_memset:
    cmp r2, #0
    bxeq    lr
.LBB0_1:
    strb    r1, [r0], #1
    subs    r2, r2, #1
    bne .LBB0_1
    bx  lr

I dont like answering my own question, can sit here for a bit so I can maybe find the answer next time or if the SO gods decide to leave it here that is fine...

Answer 2

I encounter the same problem and if it still can help anyone this is what I did to solve it - I modify the LoopIdiomRecognize.cpp file in the llvm source code: there is a code that checks if the name of the function is memset or memcpy it cancel the optimization, so I changed it from:

StringRef Name = L->getHeader()->getParent()->getName();
  if (Name == "memset" || Name == "memcpy")

to

StringRef Name = L->getHeader()->getParent()->getName();
  if (Name.endswith("memset") || Name == "memcpy")