[lld][COFF] Add /discard-section option to discard input sections by name (#189542)

This provides a general mechanism similar to ELF linker scripts'
/DISCARD/ for COFF. Though the intention is to explicitly discard
.llvmbc and .llvmcmd sections. (See discussion in #150897, #188398
for more details.)

lld/COFF/Config.h

@@ -216,6 +216,9 @@ struct Configuration {
   // Used for /merge:from=to (e.g. /merge:.rdata=.text)
   std::map<StringRef, StringRef> merge;
 
+  // Used for /discard-section:.name
+  llvm::StringSet<> discardSection;
+
   // Used for /section=.name,{DEKPRSW} to set section attributes.
   std::map<StringRef, uint32_t> section;
   // Used for /sectionlayout: to layout sections in specified order.

lld/COFF/Driver.cpp

@@ -2103,6 +2103,10 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   for (auto *arg : args.filtered(OPT_merge))
     parseMerge(arg->getValue());
 
+  // Handle /discard-section
+  for (auto *arg : args.filtered(OPT_discard_section))
+    config->discardSection.insert(arg->getValue());
+
   // Add default section merging rules after user rules. User rules take
   // precedence, but we will emit a warning if there is a conflict.
   parseMerge(".idata=.rdata");

lld/COFF/InputFiles.cpp

@@ -404,6 +404,9 @@ SectionChunk *ObjFile::readSection(uint32_t sectionNumber,
     return nullptr;
   }
 
+  if (symtab.ctx.config.discardSection.contains(name))
+    return nullptr;
+
   // Object files may have DWARF debug info or MS CodeView debug info
   // (or both).
   //

lld/COFF/Options.td

@@ -45,6 +45,7 @@ def color_diagnostics_eq: Joined<["--"], "color-diagnostics=">,
 def defaultlib : P<"defaultlib", "Add the library to the list of input files">;
 def delayload : P<"delayload", "Delay loaded DLL name">;
 def diasdkdir : P<"diasdkdir", "Set the location of the DIA SDK">;
+def discard_section : P<"discard-section", "Discard input sections by name">;
 def dwodir : P<"dwodir",
     "Directory to store .dwo files when LTO and debug fission are used">;
 def entry   : P<"entry", "Name of entry point symbol">;

lld/test/COFF/discard-section.test

@@ -0,0 +1,51 @@
+# RUN: yaml2obj %s -o %t.obj
+
+## By default, all sections are preserved.
+# RUN: lld-link /entry:main /subsystem:console /out:%t.exe %t.obj
+# RUN: llvm-readobj -S %t.exe | FileCheck --check-prefix=PRESERVE %s
+
+# PRESERVE: Name: .rdata
+# PRESERVE: Name: .llvmbc
+# PRESERVE: Name: .llvmcmd
+
+## /discard-section discards sections by name.
+# RUN: lld-link /entry:main /subsystem:console /discard-section:.llvmbc /discard-section:.llvmcmd /out:%t-stripped.exe %t.obj
+# RUN: llvm-readobj -S %t-stripped.exe | FileCheck --check-prefix=NOBC %s
+
+# NOBC:     Name: .rdata
+# NOBC-NOT: Name: .llvmbc
+# NOBC-NOT: Name: .llvmcmd
+
+# RUN: lld-link /entry:main /subsystem:console /discard-section:.rdata /out:%t-nordata.exe %t.obj
+# RUN: llvm-readobj -S %t-nordata.exe | FileCheck --check-prefix=NORDATA %s
+
+# NORDATA-NOT: Name: .rdata
+# NORDATA:     Name: .llvmbc
+# NORDATA:     Name: .llvmcmd
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    SectionData:     "C3"
+  - Name:            .rdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    SectionData:     "00000000"
+  - Name:           .llvmbc
+    Characteristics: [ IMAGE_SCN_MEM_DISCARDABLE ]
+    SectionData:     "4243C0DE"
+  - Name:           .llvmcmd
+    Characteristics: [ IMAGE_SCN_MEM_DISCARDABLE ]
+    SectionData:     "2D63633100"
+
+symbols:
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...