Condition execution optimization with gcc 7.5

[Benjamin Minguez via Gcc-help <gcc-help@xxxxxxxxxxx>]

Hello everyone,

I'm trying to optimize an application that contains a lot of branches. I'm targeting armv8 processors and I'm using GCC 7.5.0 for compatibility reason.
As the original application is similar to NGINX, I investigated on NGINX. I'm focusing on the HTTP header parsing. Basically, the algorithm parse byte per byte and based on the value stores some variables.
Here is an example, /src/http/ngx_http_parse.c: ngx_http_parse_header_line
                if (c) {
                    hash = ngx_hash(0, c);
                    r->lowcase_header[0] = c;
                    i = 1;
                    break;
                }

                if (ch == '_') {
                    if (allow_underscores) {
                        hash = ngx_hash(0, ch);
                        r->lowcase_header[0] = ch;
                        i = 1;

                    } else {
                        r->invalid_header = 1;
                    }

                    break;
                }
Also, most of branches are not predictable because it compares against data coming from the network.
>From these observations, I looked at the conditional execution optimization step in GCC and I found this function that should do the work: cond_exec_find_if_block. And how to customize the decision to use conditional instructions:
                #define MAX_CONDITIONAL_EXECUTE arm_max_conditional_execute ()
                int
                arm_max_conditional_execute (void)
                {
                  return max_insns_skipped;
                }
                static int max_insns_skipped = 5;

I tried to compile NGNIX in -O2 (that should enable if-conversion2) but I did not noticed any change in the code. I enable GCC debug (-da) and also add some debug in this function and I figure out that targetm.have_conditional_execution is set to false.

First, do you how to switch this variable to true. I guess it is an option during the configuration step of GCC.
Then, I know  that the decision to use conditional execution is based on the extra cost added to compute both branches compare to the cost of a branch. In this specific case, branches are miss predicted and the cost is, indeed, high. Do you think that increasing the max_insns_skipped will be enough to help GCC to use conditional execution?

Thank you in advance for your answers.

Best,
Benjamin Minguez