[lld/mac] Make X86_64::getImplicitAddend not do heap allocations

[thakis]

Summary

Speeds up linking Chromium's base_unittests almost 10%. According to ministat:

    N           Min           Max        Median           Avg        Stddev
x   5    0.72193289    0.73073196    0.72560811    0.72565799  0.0032265649
+   5    0.64069581    0.67173195    0.65876389    0.65796089   0.011349451
Difference at 95.0% confidence
	-0.0676971 +/- 0.0121682
	-9.32906% +/- 1.67685%
	(Student's t, pooled s = 0.00834328)

[smeenai]

LGTM, thanks!

CC @gkm there might be something similar in the outstanding arm64 diff.

[int3]

oh wow... I'd have thought that compilers would be better at optimizing such a common operation

[thakis]

Thanks!

In D92734#2436805, @int3 wrote:

oh wow... I'd have thought that compilers would be better at optimizing such a common operation

The compiler would have to inline validateLength() to do that, and it's called a few times and has a large-ish loop at the bottom. A common technique is to manually outline the cold part of a function, and then the compiler can inline the hot part and that might be enough to let the compiler optimize this (...but I haven't checked).

lld/MachO/Arch/X86_64.cpp

static std::string getErrorLocation(MemoryBufferRef mb, const section_64 &sec,
   return ("invalid relocation at offset " + std::to_string(rel.r_address) +
           " of " + sec.segname + "," + sec.sectname + " in " +
           mb.getBufferIdentifier())
       .str();
 }
 
 static void validateLength(MemoryBufferRef mb, const section_64 &sec,
                            const relocation_info &rel,
-                           const std::vector<uint8_t> &validLengths) {
-  if (std::find(validLengths.begin(), validLengths.end(), rel.r_length) !=
-      validLengths.end())
+                           ArrayRef<uint8_t> validLengths) {
+  if (find(validLengths, rel.r_length) != validLengths.end())
     return;
 
   std::string msg = getErrorLocation(mb, sec, rel) + ": relocations of type " +
                     std::to_string(rel.r_type) + " must have r_length of ";
   bool first = true;
   for (uint8_t length : validLengths) {
     if (!first)
       msg += " or ";