Guessing output with built-in usage.

Prompt

Guess the output of the following code. Remember to recall what you can remember, and perform research using ONLY search tools if your tools exist. Explain the final output, how would the code works, and how would immediate internal state looks like (both high level and low level representation). Format your final answer as HTML/CSS. ``` #include <stdint.h> #include <stdlib.h> #include <string.h> #include <stdio.h> #if defined(_MSC_VER) #define ATTR_FORCE_INLINE __forceinline #define ATTR_NEVER_INLINE __declspec(noinline) #elif defined(__GNUC__) || defined(__clang__) #define ATTR_FORCE_INLINE inline __attribute__((__always_inline__)) #define ATTR_NEVER_INLINE __attribute__((noinline)) #else #define ATTR_FORCE_INLINE inline #define ATTR_NEVER_INLINE #endif #if defined(__GNUC__) || defined(__clang__) #define ATTR_OA __attribute__((optimize( #define ATTR_OZ ))) #define ATTR_OPTIMIZE(NAME) __attribute__((optimize(ATTR_O_##NAME))) #elif defined(_MSC_VER) #define ATTR_OA __pragma(optimize( #define ATTR_OZ , on)) #define ATTR_OPTIMIZE(NAME) __pragma(optimize(ATTR_O_##NAME, on)) #else #define ATTR_OPTIMIZE #endif struct ctx; typedef struct ctx*(*ctx_fn)(struct ctx*); #if defined(__GNUC__) || defined(__clang__) #define jump_addr (__builtin_frame_address(0)) #define jump_data(addr) ((void**)(addr))[1] #elif defined(_MSC_VER) #define jump_addr (_AddressOfReturnAddress()) #define jump_data(addr) *((void**)(addr)) #include <malloc.h> #define alloca _alloca #else #error "Unsupported compiler." #endif enum ctx_kind { CTX_KIND_CALL = 0, CTX_KIND_CODE = 1, CTX_KIND_COMP = 2, CTX_KIND_JUMP = 3 }; struct ctx { union { struct { struct ctx *link; ctx_fn func; void **addr; } state; // 0 1 2 struct { struct ctx *comp; struct ctx **path; } track; // 3 }; enum ctx_kind kind; }; struct ctx* ctx_jump(struct ctx* link, struct ctx *comp, struct ctx *code) { struct ctx ctx = {0}; ctx.track.comp = comp; ctx.kind = CTX_KIND_JUMP; size_t count = 1; struct ctx *curr = comp; while (!(curr == code || curr == NULL)) { curr = curr->state.link; ++ count; } ctx.track.path = alloca(sizeof(struct ctx*) * count); curr = comp; for (size_t i = count; i --> 0; curr = curr->state.link) ctx.track.path[i] = curr; ctx.track.path[0]->state.func(&ctx); return link; } ATTR_NEVER_INLINE ATTR_OA "-fno-omit-frame-pointer" ATTR_OZ struct ctx* ctx_form(struct ctx *link, void *data, void*(*init)(void*, struct ctx*)) { if (link->kind == CTX_KIND_JUMP) { struct ctx *ctx = *(++ link->track.path); jump_data(jump_addr) = jump_data(ctx->state.addr); if (ctx->kind == CTX_KIND_COMP) link = ctx->state.link; else return ctx->state.func(link)->state.link; } else ((void**)((uintptr_t)link + sizeof(struct ctx)))[1] = init == NULL ? data : init(data, link); return link; } #define CTX_TMPL(KIND, LINK) \ void *ctx = alloca(sizeof(void*) * 2 + sizeof(struct ctx)); \ ((struct ctx*)ctx)->state.link = LINK; \ ((struct ctx*)ctx)->state.func = func; \ ((struct ctx*)ctx)->state.addr = jump_addr; \ ((struct ctx*)ctx)->kind = KIND; \ *(void**)((uintptr_t)ctx + sizeof(struct ctx)) = pass; \ return func(ctx)->state.link; #define CTX_PARAM_BASE ctx_fn func, void *pass #define CTX_PARAM struct ctx *link, CTX_PARAM_BASE #define CTX_FUNC(NAME, PARAM, KIND, LINK) \ ATTR_OA "-fno-omit-frame-pointer" ATTR_OZ struct ctx* ctx_##NAME(PARAM) { CTX_TMPL(KIND, LINK); } CTX_FUNC(call, CTX_PARAM, CTX_KIND_CALL, link) CTX_FUNC(comp, CTX_PARAM, CTX_KIND_COMP, link) CTX_FUNC(code, CTX_PARAM, CTX_KIND_CODE, link) CTX_FUNC(main, CTX_PARAM_BASE, CTX_KIND_CODE, NULL) #undef CTX_FUNC #undef CTX_PARAM #undef CTX_PARAM_BASE #undef CTX_TMPL struct ctx* C_fn(struct ctx*); struct ctx* B_fn(struct ctx*); struct ctx* A_fn(struct ctx*); struct ctx* b_fn(struct ctx*); struct ctx* a_fn(struct ctx*); struct ctx* main_fn(struct ctx*); ATTR_OA "-fno-ipa-cp-clone" ATTR_OZ struct ctx* scope_fn(struct ctx*); void ctx_dump(struct ctx *link, char *msg) { struct ctx *temp = link; printf("\n===== %s ; ctx_dump =====\n", msg); size_t guard = 0; do { printf("---\n"); printf("curr=%p\n", temp); if (temp->kind == CTX_KIND_JUMP) { printf("comp=%p\n", temp->track.comp); printf("path=%p\n", temp->track.path); } else { if (temp->state.func == (ctx_fn)main_fn) printf("func=main_fn\n"); else if (temp->state.func == (ctx_fn)scope_fn) printf("func=scope_fn\n"); else if (temp->state.func == (ctx_fn)a_fn) printf("func=a_fn\n"); else if (temp->state.func == (ctx_fn)b_fn) printf("func=b_fn\n"); else if (temp->state.func == (ctx_fn)A_fn) printf("func=A_fn\n"); else if (temp->state.func == (ctx_fn)B_fn) printf("func=B_fn\n"); else if (temp->state.func == (ctx_fn)C_fn) printf("func=C_fn\n"); else printf("func=unknown, %p\n", temp->state.func); printf("addr=%p\n", temp->state.addr); } temp = temp->state.link; } while (temp->state.link != NULL && guard++ < 100); printf("====================\n"); } void* evt_fn(void *data, struct ctx *link) { printf("! %s\n", (char*)data); return NULL; } static struct ctx *mmm = NULL; static struct ctx *ddd = NULL; struct ctx* C_fn(struct ctx* link) { link = ctx_form(link, NULL, 0); printf("------- 13: %p\n", link); link = ctx_jump(link, mmm, ddd); printf("------- 14: %p\n", link); return link; } struct ctx* B_fn(struct ctx* link) { link = ctx_form(link, "B_fn", evt_fn); printf("------ 12: %p\n", link); link = ctx_call(link, C_fn, NULL); printf("------ 15: %p\n", link); return link; } struct ctx* A_fn(struct ctx* link) { if (mmm == NULL) mmm = link; printf("----- 10: %p\n", link); link = ctx_form(link, "A_fn", evt_fn); printf("----- 11: %p\n", link); link = ctx_call(link, B_fn, NULL); printf("----- 16: %p\n", link); return link; } struct ctx* b_fn(struct ctx* link) { printf("---- 8: %p\n", link); link = ctx_form(link, "b_fn", evt_fn); printf("---- 9: %p\n", link); link = ctx_comp(link, A_fn, NULL); printf("---- 17: %p\n", link); return link; } struct ctx* a_fn(struct ctx* link) { printf("--- 6: %p\n", link); link = ctx_form(link, "a_fn", evt_fn); printf("--- 7: %p\n", link); link = ctx_call(link, b_fn, NULL); printf("--- 18: %p\n", link); return link; } struct ctx* scope_fn(struct ctx* link) { if (ddd == NULL) ddd = link; printf("-- 4: %p\n", link); link = ctx_form(link, "scope_fn", evt_fn); printf("-- 5: %p\n", link); link = ctx_call(link, a_fn, NULL); printf("-- 19: %p\n", link); return link; } struct ctx* main_fn(struct ctx *link) { printf("- 2: %p\n", link); link = ctx_form(link, "main_fn", evt_fn); printf("- 3: %p\n", link); link = ctx_code(link, scope_fn, NULL); printf("- 20: %p\n", link); return link; } int main(void) { char *pf_buf = malloc(sizeof(char) * BUFSIZ); setvbuf(stdout, pf_buf, _IONBF, BUFSIZ); printf("1\n"); ctx_main((ctx_fn)main_fn, NULL); printf("21\n"); free(pf_buf); return 0; } // gcc ./main.c -O3 -o main ```

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