[LLD][ELF] - Make compression level be dependent on -On.

Currently LLD always use zlib compression level 6.
This patch changes it to use 1 for -O0, -O1 and 6 for -O2.

It fixes https://bugs.llvm.org/show_bug.cgi?id=44089.

There was also a thread in llvm-dev on this topic:
https://lists.llvm.org/pipermail/llvm-dev/2018-August/125020.html

Here is a table with results of building clang mentioned there:

```
Level   Time            Size
0       0m17.128s       2045081496   Z_NO_COMPRESSION
1       0m31.471s       922618584    Z_BEST_SPEED
2       0m32.659s       903642376
3       0m36.749s       890805856
4       0m41.532s       876697184
5       0m48.383s       862778576
6       1m3.176s        855251640    Z_DEFAULT_COMPRESSION
7       1m15.335s       853755920
8       2m0.561s        852497560
9       2m33.972s       852397408    Z_BEST_COMPRESSION
```

It shows that it is probably not reasonable to use values greater than 6.

Differential revision: https://reviews.llvm.org/D70658

1 files changed, 6 insertions, 1 deletions

diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index cc051dba0e0..8d328626b85 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -272,7 +272,12 @@ template <class ELFT> void OutputSection::maybeCompress() {
   // Write section contents to a temporary buffer and compress it.
   std::vector<uint8_t> buf(size);
   writeTo<ELFT>(buf.data());
-  if (Error e = zlib::compress(toStringRef(buf), compressedData))
+  // We chose 1 as the default compression level because it is the fastest. If
+  // -O2 is given, we use level 6 to compress debug info more by ~15%. We found
+  // that level 7 to 9 doesn't make much difference (~1% more compression) while
+  // they take significant amount of time (~2x), so level 6 seems enough.
+  if (Error e = zlib::compress(toStringRef(buf), compressedData,
+                               config->optimize >= 2 ? 6 : 1))
     fatal("compress failed: " + llvm::toString(std::move(e)));
 
   // Update section headers.