Changeset 11801:80af6a1e6beb

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x86: fixed branching() computation for branch uops that only changes nupc and not npc
When a branch micro-op belongs to a flow and the micro-op does not change the nPC and just updates the nuPC

(like a 'rep movs' flow), branching() function always returns not-taken no matter  actual

micro-branch outcome. Provided fix adds to the equation  nuPC attribute checking since these kind of branch

micro-op only updates that pointer.
This issue has been found while debugging the performance of a copy-loop implemented with memcopy function. Without the fix, 'rep movss' internal micro-branch was always predicted as not-taken causing an squash event after every branch micro-branch execution.
Using the provided test, branch mispredition went from 1922 without the fix to 7.

I used this command line to evaluate the performance:

./build/X86/gem5.opt configs/example/se.py --cpu-type=detailed --caches --l2cache -c /path/to/binary


This is the source code of the binary:
#include <string.h>
#include "m5op.h"

#define SIZE 15*1024

//arrays are cache aligned
char a [SIZE] __attribute__((aligned(0x40)));
char b [SIZE] __attribute__((aligned(0x40)));

int main ()
{
    //some warmup
    int i;
    for (i = 0; i < SIZE; ++i)
    {
        a[i] = 1;
        b[i] = 2;
    }

    m5_reset_stats(0, 0);
    memcpy(a, b, SIZE);
    m5_exit(0);

    //keep compiler happy
    return 0;
}


Which was compiled with this makefile:

GEM5_PATH=/path/to/gem5

binary: binary.c $(GEM5_PATH)/util/m5/m5op_x86.S
    gcc -o $@ $^ -static -I$(GEM5_PATH)/util/m5