[ELF] Adjust --compress-sections to support compression level

zstd excels at scaling from low-ratio-very-fast to
high-ratio-pretty-slow. Some users prioritize speed and prefer disk read
speed, while others focus on achieving the highest compression ratio
possible, similar to traditional high-ratio codecs like LZMA.

Add an optional `level` to `--compress-sections` (#84855) to cater to
these diverse needs. While we initially aimed for a one-size-fits-all
approach, this no longer seems to work.
(https://richg42.blogspot.com/2015/11/the-lossless-decompression-pareto.html)

When --compress-debug-sections is used together, make
--compress-sections take precedence since --compress-sections is usually
more specific.

Remove the level distinction between -O/-O1 and -O2 for
--compress-debug-sections=zlib for a more consistent user experience.

Pull Request: https://github.com/llvm/llvm-project/pull/90567

4 files changed, 26 insertions, 16 deletions

diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 33bfa42..c55b547 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -224,7 +224,8 @@ struct Config {
   bool checkSections;
   bool checkDynamicRelocs;
   std::optional<llvm::DebugCompressionType> compressDebugSections;
-  llvm::SmallVector<std::pair<llvm::GlobPattern, llvm::DebugCompressionType>, 0>
+  llvm::SmallVector<
+      std::tuple<llvm::GlobPattern, llvm::DebugCompressionType, unsigned>, 0>
       compressSections;
   bool cref;
   llvm::SmallVector<std::pair<llvm::GlobPattern, uint64_t>, 0>
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index a5b47f0..b29e1e1 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -1533,9 +1533,17 @@ static void readConfigs(opt::InputArgList &args) {
             ": parse error, not 'section-glob=[none|zlib|zstd]'");
       continue;
     }
-    auto type = getCompressionType(fields[1], arg->getSpelling());
+    auto [typeStr, levelStr] = fields[1].split(':');
+    auto type = getCompressionType(typeStr, arg->getSpelling());
+    unsigned level = 0;
+    if (fields[1].size() != typeStr.size() &&
+        !llvm::to_integer(levelStr, level)) {
+      error(arg->getSpelling() +
+            ": expected a non-negative integer compression level, but got '" +
+            levelStr + "'");
+    }
     if (Expected<GlobPattern> pat = GlobPattern::create(fields[0])) {
-      config->compressSections.emplace_back(std::move(*pat), type);
+      config->compressSections.emplace_back(std::move(*pat), type, level);
     } else {
       error(arg->getSpelling() + ": " + toString(pat.takeError()));
       continue;
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
index 72eaf15..73a4f96 100644
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -68,8 +68,9 @@ defm compress_debug_sections:
   MetaVarName<"[none,zlib,zstd]">;
 
 defm compress_sections: EEq<"compress-sections",
-  "Compress non-SHF_ALLOC output sections matching <section-glob>">,
-  MetaVarName<"<section-glob>=[none|zlib|zstd]">;
+  "Compress output sections that match the glob and do not have the SHF_ALLOC flag."
+  "The compression level is <level> (if specified) or a default speed-focused level">,
+  MetaVarName<"<section-glob>={none,zlib,zstd}[:level]">;
 
 defm defsym: Eq<"defsym", "Define a symbol alias">, MetaVarName<"<symbol>=<value>">;
 
diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index 1b09e5b..2dbbff0 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -339,12 +339,13 @@ template <class ELFT> void OutputSection::maybeCompress() {
   (void)sizeof(Elf_Chdr);
 
   DebugCompressionType ctype = DebugCompressionType::None;
-  for (auto &[glob, t] : config->compressSections)
-    if (glob.match(name))
-      ctype = t;
+  unsigned level = 0; // default compression level
   if (!(flags & SHF_ALLOC) && config->compressDebugSections &&
       name.starts_with(".debug_") && size)
     ctype = *config->compressDebugSections;
+  for (auto &[glob, t, l] : config->compressSections)
+    if (glob.match(name))
+      std::tie(ctype, level) = {t, l};
   if (ctype == DebugCompressionType::None)
     return;
   if (flags & SHF_ALLOC) {
@@ -376,13 +377,14 @@ template <class ELFT> void OutputSection::maybeCompress() {
   auto shardsOut = std::make_unique<SmallVector<uint8_t, 0>[]>(numShards);
 
 #if LLVM_ENABLE_ZSTD
-  // Use ZSTD's streaming compression API which permits parallel workers working
-  // on the stream. See http://facebook.github.io/zstd/zstd_manual.html
-  // "Streaming compression - HowTo".
+  // Use ZSTD's streaming compression API. See
+  // http://facebook.github.io/zstd/zstd_manual.html "Streaming compression -
+  // HowTo".
   if (ctype == DebugCompressionType::Zstd) {
     parallelFor(0, numShards, [&](size_t i) {
       SmallVector<uint8_t, 0> out;
       ZSTD_CCtx *cctx = ZSTD_createCCtx();
+      ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, level);
       ZSTD_inBuffer zib = {shardsIn[i].data(), shardsIn[i].size(), 0};
       ZSTD_outBuffer zob = {nullptr, 0, 0};
       size_t size;
@@ -410,12 +412,10 @@ template <class ELFT> void OutputSection::maybeCompress() {
 
 #if LLVM_ENABLE_ZLIB
   // We chose 1 (Z_BEST_SPEED) 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.
+  // fast and provides decent compression ratios.
   if (ctype == DebugCompressionType::Zlib) {
-    const int level = config->optimize >= 2 ? 6 : Z_BEST_SPEED;
+    if (!level)
+      level = Z_BEST_SPEED;
 
     // Compress shards and compute Alder-32 checksums. Use Z_SYNC_FLUSH for all
     // shards but the last to flush the output to a byte boundary to be