[llvm-dev] assembly code for array iteration generated by llvm is much slower than gcc

Lori Yao Yu via llvm-dev llvm-dev at lists.llvm.org
Sat Apr 25 20:37:20 PDT 2020

Hi all developers,

    I'm changing compiler from gcc to llvm on a RISCV target now. but I found in some case the assembly code generated by llvm is much more than gcc. It cause my program's performance about 40% decrease.

   The flowing is a simple test code. It shows the problem. We can see than gcc prefer to use pointer to iterate the array,  but llvm perfere to use index to iterate the array. So llvm generate more codes to calculate the memory address of an array element from the index.

    My question is that if there's an option of llvm, to let it iterate array by pointer. Or it’s a bug of llvm not resolved now ?

        Test C code:

Int func(int w1, int w2, int *b, int *c) {

   Int wstart = 0;

    Int i = 0;

   Int j = 0;

   Int sum = 0;

   In wend = 0;

   Int dst_idx = 0;

   Int dst_idx2 = 0;

   for (I = 0; I < w2; i++) {

        wstart = i * w1;

        wend = i / w1;

        sum = c[wstart];

        for (j = wstart + 1; j < wend; j++) {

                         sum += c[j * w2];

           sum += c[j * w1];


       dst_idx = w1 * i + w2;

       dst_idx2 = w2 * i + w1;

       b[dst_idx] = sum;

       b[dst_idx2] = sum/2;



Compile command:

riscv32-unkown-elf-g++ -nostartfiles -nostdlib -O2 -march=rv32imf -mabi=ilp32f -fno-builtin -S perf.c -o perf.g++

clang++ -O2 –target=riscv32 -march=rv32img -mabi=ilp32f -nostdlib -fno-builtin -S perf.c -o perf.lang

the gcc version is 7.2.0

the llvm version is 10.0.0

Assembly code of the loop generated by gcc and llvm:

[cid:image002.jpg at 01D61BBF.0A4250B0]

   Thank you all for your time and any help you can provide.


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