[ELF] Retain uncompressed if compressed content is larger

--compress-debug-sections in GNU ld, gas, and LLVM integrated assembler
retain the uncompressed content if the compressed content is larger.

This patch also updates the manpage (-O2 does not enable zlib level 6)
and fixes a crash of --compress-sections when the uncompressed section
is empty.

Author: MaskRay

lld/ELF/Options.td

@@ -69,6 +69,7 @@ defm compress_debug_sections:
 
 defm compress_sections: EEq<"compress-sections",
   "Compress output sections that match the glob and do not have the SHF_ALLOC flag. "
+  "The sections remain uncompressed if compressed content would be larger. "
   "The compression level is <level> (if specified) or a default speed-focused level">,
   MetaVarName<"<section-glob>={none,zlib,zstd}[:level]">;
 

lld/ELF/OutputSections.cpp

@@ -344,9 +344,10 @@ template <class ELFT> void OutputSection::maybeCompress() {
   (void)sizeof(Elf_Chdr);
 
   DebugCompressionType ctype = DebugCompressionType::None;
+  size_t compressedSize = sizeof(Elf_Chdr);
   unsigned level = 0; // default compression level
   if (!(flags & SHF_ALLOC) && config->compressDebugSections &&
-      name.starts_with(".debug_") && size)
+      name.starts_with(".debug_"))
     ctype = *config->compressDebugSections;
   for (auto &[glob, t, l] : config->compressSections)
     if (glob.match(name))
@@ -360,7 +361,6 @@ template <class ELFT> void OutputSection::maybeCompress() {
   }
 
   llvm::TimeTraceScope timeScope("Compress sections");
-  compressed.uncompressedSize = size;
   auto buf = std::make_unique<uint8_t[]>(size);
   // Write uncompressed data to a temporary zero-initialized buffer.
   {
@@ -378,7 +378,6 @@ template <class ELFT> void OutputSection::maybeCompress() {
   [[maybe_unused]] constexpr size_t shardSize = 1 << 20;
   auto shardsIn = split(ArrayRef<uint8_t>(buf.get(), size), shardSize);
   const size_t numShards = shardsIn.size();
-  compressed.numShards = numShards;
   auto shardsOut = std::make_unique<SmallVector<uint8_t, 0>[]>(numShards);
 
 #if LLVM_ENABLE_ZSTD
@@ -409,9 +408,8 @@ template <class ELFT> void OutputSection::maybeCompress() {
       shardsOut[i] = std::move(out);
     });
     compressed.type = ELFCOMPRESS_ZSTD;
-    size = sizeof(Elf_Chdr);
     for (size_t i = 0; i != numShards; ++i)
-      size += shardsOut[i].size();
+      compressedSize += shardsOut[i].size();
   }
 #endif
 
@@ -434,18 +432,23 @@ template <class ELFT> void OutputSection::maybeCompress() {
 
     // Update section size and combine Alder-32 checksums.
     uint32_t checksum = 1;       // Initial Adler-32 value
-    size = sizeof(Elf_Chdr) + 2; // Elf_Chdir and zlib header
+    compressedSize += 2;         // Elf_Chdir and zlib header
     for (size_t i = 0; i != numShards; ++i) {
-      size += shardsOut[i].size();
+      compressedSize += shardsOut[i].size();
       checksum = adler32_combine(checksum, shardsAdler[i], shardsIn[i].size());
     }
-    size += 4; // checksum
+    compressedSize += 4; // checksum
     compressed.type = ELFCOMPRESS_ZLIB;
     compressed.checksum = checksum;
   }
 #endif
 
+  if (compressedSize >= size)
+    return;
+  compressed.uncompressedSize = size;
   compressed.shards = std::move(shardsOut);
+  compressed.numShards = numShards;
+  size = compressedSize;
   flags |= SHF_COMPRESSED;
 }
 

lld/docs/ld.lld.1

@@ -148,6 +148,7 @@ Alias for
 .Fl -color-diagnostics Ns = Ns Cm auto .
 .It Fl -compress-debug-sections Ns = Ns Ar value
 Compress DWARF debug sections.
+The sections remain uncompressed if compressed content would be larger.
 .Cm value
 may be
 .Pp
@@ -163,6 +164,7 @@ Use the default compression level in zstd.
 .Pp
 .It Fl -compress-sections Ns = Ns Ar section-glob={none,zlib,zstd}[:level]
 Compress output sections that match the glob and do not have the SHF_ALLOC flag.
+The matched sections remain uncompressed if compressed content would be larger.
 The compression level is
 .Cm level
 (if specified) or a default speed-focused level.
@@ -420,9 +422,7 @@ Disable string merging.
 .It Cm 1
 Enable string merging.
 .It Cm 2
-Enable string tail merging. If
-.Fl -compress-debug-sections
-is given, compress debug sections at compression level 6 instead of 1.
+Enable string tail merging.
 .El
 .Pp
 .Fl O Ns Cm 1

lld/test/ELF/compress-debug-sections-zstd.s

@@ -21,7 +21,7 @@
 
 # OUTPUT-SEC:      .debug_str    PROGBITS [[#%x,]] [[#%x,]] [[#%x,]] 01 MSC 0 0  1
 # OUTPUT-SEC-NEXT: .debug_frame  PROGBITS [[#%x,]] [[#%x,]] 000000   00     0 0  1
-# OUTPUT-SEC-NEXT: .debug_loc    PROGBITS [[#%x,]] [[#%x,]] [[#%x,]] 00   C 0 0  1
+# OUTPUT-SEC-NEXT: .debug_loc    PROGBITS [[#%x,]] [[#%x,]] 000010   00     0 0  1
 
 .section .debug_str,"MS",@progbits,1
 .LASF2:

lld/test/ELF/compress-sections.s

@@ -25,8 +25,8 @@
 # CHECK2-NEXT: foo1       PROGBITS [[#%x,FOO1:]]    [[#%x,]] [[#%x,]] 00 A    0   0  8
 # CHECK2-NEXT: .text      PROGBITS [[#%x,TEXT:]]    [[#%x,]] [[#%x,]] 00 AX   0   0  4
 # CHECK2:      nonalloc0  PROGBITS 0000000000000000 [[#%x,]] [[#%x,]] 00 C    0   0  1
-# CHECK2-NEXT: nonalloc1  PROGBITS 0000000000000000 [[#%x,]] [[#%x,]] 00      0   0  8
-# CHECK2-NEXT: smallc0    PROGBITS 0000000000000000 [[#%x,]] [[#%x,]] 00 C    0   0  1
+# CHECK2-NEXT: nonalloc1  PROGBITS 0000000000000000 [[#%x,]] 000088   00      0   0  8
+# CHECK2-NEXT: smallc0    PROGBITS 0000000000000000 [[#%x,]] 00000c   00      0   0  1
 # CHECK2-NEXT: .debug_str PROGBITS 0000000000000000 [[#%x,]] [[#%x,]] 01 MSC  0   0  1
 
 # CHECK2: 0000000000000090  0 NOTYPE  LOCAL  DEFAULT   [[#]] (nonalloc0) sym0