make-0001: implicit.c pattern_search file->deps O(R×D×F) → O(R×D+F)

Defect

File: src/implicit.c
Function: pattern_search
Lines: ~796 (Savannah HEAD, 2025-03-27)
CWE: 407 — Inefficient Algorithmic Complexity
Severity: HIGH

Pattern

/* Outer: for (intermed_ok = 0; intermed_ok < 2; ++intermed_ok) */
/* Outer: for (ri = 0; ri < nrules; ri++)                        */
/*   Middle: while(1) / for (d = dl; d != 0; d = d->next)        */

                  if (df && df->is_target)
                    explicit = 1;
                  else
                    for (dp = file->deps; dp != 0; dp = dp->next)  /* O(F) */
                      if (streq (d->name, dep_name (dp)))           /* O(F) linear scan */
                        break;

Complexity

R = number of pattern rules (can be hundreds of built-in + user-defined rules)
D = number of pattern prerequisites per rule (typically 1-5)
F = number of explicit prerequisites of the current target file

For each target, pattern_search runs through R rules × D deps × F explicit deps. The O(F) inner linear scan is performed for every (rule, dep) pair.

Worst case: target with F=100 deps and R=200 rules with D=5 deps each:

Without fix: 200 × 5 × 100 = 100,000 string comparisons
With fix: build hash set once from F deps (O(F)), then O(1) per lookup: 200 × 5 × 1 = 1,000 lookups + 100 hash inserts = ~1,100 ops
Speedup: ~91× at F=100, scales as O(F)

Root Cause

The function pattern_search is the hot path called for every target that needs an implicit rule. In a project with many targets (e.g. a large parallel make with hundreds of object files, each with many explicit prerequisites), this inner loop fires F times per (rule, dep) combination.

file->deps is a linked list — no random access or hashing. The membership test streq(d->name, dep_name(dp)) scans the whole list every time.

Fix

Build a hash_set (or a struct hash_table using GNU Make's hashmap infrastructure) from file->deps once before the rule loop begins. Replace the inner for (dp = file->deps; ...) linear scan with an O(1) hash lookup.

/* CWE-407 fix: build a set of explicit dep names once before the rule loop.
   The inner for(dp = file->deps; ...) scan was O(F) per (rule, dep) pair,
   giving O(R * D * F) total.  A hash set gives O(R * D + F). */
struct hash_table *dep_name_set = make_file_dep_name_set (file);

/* ... inside the rule/dep loops ... */

  if (df && df->is_target)
    explicit = 1;
  else
    dp = dep_name_set_contains (dep_name_set, d->name) ? (struct dep *)1 : 0;

GNU Make already uses struct hash_table (see hash.h) for file name lookup (hash_find_item). The same infrastructure applies here.

Evidence

Confirmed present in:

src/implicit.c in GNU Make mirror (mirror/make on GitHub, ~2024)
src/implicit.c in GNU Make Savannah HEAD (cgit/make.git, 2025-03-27)

Lines 796-797 (Savannah):

                    for (dp = file->deps; dp != 0; dp = dp->next)
                      if (streq (d->name, dep_name (dp)))

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