[WebAssembly] Don't compress LEBs by default

[sbc100]

Summary

LEB compression breaks debug info so we don't want to enable
it by default, even at high optimization levels.

[sbc100]

Perhaps I should make this -z compress although I'm not sure what the policy is in using -z keywords vs regular flags.

[sbc100]

• add check for -r

[yurydelendik]

lgtm

[ruiu]

lld/trunk/wasm/Driver.cpp

398 ↗

(On Diff #161300)

All Config members that correspond to command line options have the same name as the command line options, so this should be Config->CompressRelocations.

test/wasm/compress-relocs.ll

 ; RUN: llc -filetype=obj %p/Inputs/call-indirect.ll -o %t2.o
 ; RUN: llc -filetype=obj %s -o %t.o
 ; RUN: wasm-ld -o %t.wasm %t2.o %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
-
-; RUN: wasm-ld -O2 -o %t-compressed.wasm %t2.o %t.o
+; RUN: wasm-ld -O2 -o %t-opt.wasm %t2.o %t.o
+; RUN: obj2yaml %t-opt.wasm | FileCheck %s
+; RUN: not wasm-ld --compress-relocations -o %t-compressed.wasm %t2.o %t.o 2>&1 | FileCheck %s -check-prefix=ERROR
+; RUN: wasm-ld --strip-debug --compress-relocations -o %t-compressed.wasm %t2.o %t.o
 ; RUN: obj2yaml %t-compressed.wasm | FileCheck %s -check-prefix=COMPRESS
 
 target triple = "wasm32-unknown-unknown-wasm"
 
 define i32 @foo() {
 entry:
   ret i32 2
 }
 
 define void @_start() local_unnamed_addr {
 entry:
   ret void
 }
 
+; ERROR: wasm-ld: error: --compress-relocations is incompatible with output debug information. Please pass --strip-debug or --strip-all
+
 ; CHECK:    Body:            4100280284888080002100410028028088808000118080808000001A2000118180808000001A0B
 ; COMPRESS: Body:            41002802840821004100280280081100001A20001101001A0B

wasm/Driver.cpp

Config->MergeDataSegments =
       Args.hasFlag(OPT_merge_data_segments, OPT_no_merge_data_segments,
                    !Config->Relocatable);
   Config->PrintGcSections =
       Args.hasFlag(OPT_print_gc_sections, OPT_no_print_gc_sections, false);
   Config->SaveTemps = Args.hasArg(OPT_save_temps);
   Config->SearchPaths = args::getStrings(Args, OPT_L);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->StripDebug = Args.hasArg(OPT_strip_debug);
+  Config->CompressRelocTargets = Args.hasArg(OPT_compress_relocations);
   Config->StackFirst = Args.hasArg(OPT_stack_first);
   Config->ThinLTOCacheDir = Args.getLastArgValue(OPT_thinlto_cache_dir);
   Config->ThinLTOCachePolicy = CHECK(
       parseCachePruningPolicy(Args.getLastArgValue(OPT_thinlto_cache_policy)),
       "--thinlto-cache-policy: invalid cache policy");
   Config->ThinLTOJobs = args::getInteger(Args, OPT_thinlto_jobs, -1u);
   errorHandler().Verbose = Args.hasArg(OPT_verbose);
   ThreadsEnabled = Args.hasFlag(OPT_threads, OPT_no_threads, true);
 
   Config->InitialMemory = args::getInteger(Args, OPT_initial_memory, 0);
   Config->GlobalBase = args::getInteger(Args, OPT_global_base, 1024);
   Config->MaxMemory = args::getInteger(Args, OPT_max_memory, 0);
   Config->ZStackSize =
       args::getZOptionValue(Args, OPT_z, "stack-size", WasmPageSize);
 
-  Config->CompressRelocTargets = Config->Optimize > 0 && !Config->Relocatable;
+  if (!Config->StripDebug && !Config->StripAll && Config->CompressRelocTargets)
+    error("--compress-relocations is incompatible with output debug"
+          " information. Please pass --strip-debug or --strip-all");
 
   if (Config->LTOO > 3)
     error("invalid optimization level for LTO: " + Twine(Config->LTOO));
   if (Config->LTOPartitions == 0)
     error("--lto-partitions: number of threads must be > 0");
   if (Config->ThinLTOJobs == 0)
     error("--thinlto-jobs: number of threads must be > 0");
 
void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   if (Config->ImportTable && Config->ExportTable)
     error("--import-table and --export-table may not be used together");
 
   if (Config->Relocatable) {
     if (!Config->Entry.empty())
       error("entry point specified for relocatable output file");
     if (Config->GcSections)
       error("-r and --gc-sections may not be used together");
+    if (Config->CompressRelocTargets)
+      error("-r -and --compress-relocations may not be used together");
     if (Args.hasArg(OPT_undefined))
       error("-r -and --undefined may not be used together");
   }
 
   Symbol *EntrySym = nullptr;
   if (!Config->Relocatable) {
     llvm::wasm::WasmGlobal Global;
     Global.Type = {WASM_TYPE_I32, true};

wasm/Options.td

 
 // The follow flags are shared with the ELF linker
 def color_diagnostics: F<"color-diagnostics">,
   HelpText<"Use colors in diagnostics">;
 
 def color_diagnostics_eq: J<"color-diagnostics=">,
   HelpText<"Use colors in diagnostics; one of 'always', 'never', 'auto'">;
 
+def compress_relocations: F<"compress-relocations">,
+  HelpText<"Compress the relocation targets in the code section.">;
+
 defm demangle: B<"demangle",
     "Demangle symbol names",
     "Do not demangle symbol names">;
 
 def entry: S<"entry">, MetaVarName<"<entry>">,
   HelpText<"Name of entry point symbol">;
 
 def error_limit: J<"error-limit=">,