ld.lld: Respect LDEMULATION environment variable.

[shlevy]

I've tested this manually but haven't yet set up the repo testsuite locally.

[MaskRay]

GNU ld appears to have such a behavior but I am not sure ld.lld ever has a need for this.
In addition, ld.lld doesn't support various emulations available in ld.bfd but we seem to live well without that.
The inferred default emulation seems to work in almost all cases.

[shlevy]

@MaskRay The use case which led to this for me is building gmp. Its configure script uses ld --help to look for the auto-import string to determine if the linker supports shared linking, which doesn't work as the linker defaults to the elf driver. With this patch, I can just set LDEMULATION for the build and have configure detect correctly.

[shlevy]

Note that in nixpkgs we already do set LDEMULATION in our builds, it's just been ignored by lld up until now.

[MaskRay]

In D124954#3492089, @shlevy wrote:

@MaskRay The use case which led to this for me is building gmp. Its configure script uses ld --help to look for the auto-import string to determine if the linker supports shared linking, which doesn't work as the linker defaults to the elf driver. With this patch, I can just set LDEMULATION for the build and have configure detect correctly.

FWIW the default build of gmp works with lld and that's many distributions are already using.
If there is a missing feature, ISTM the right way is to fix the gmp build system.
Many linkers don't have the emulation concept of GNU ld. They just provide some simulation and that simulation works quite well in practice (tens of thousands of FreeBSD / Gentoo packages are buildable without LDEMULATION).

[shlevy]

Ah to be clear I only see this issue when cross-compiling to Windows with shared libs. I'd be curious to see how other setups are successfully doing that with gmp's build system.

Anyway will close if this is not wanted.

lld/ELF/Driver.cpp

static void readConfigs(opt::InputArgList &args) {
   // the base address. However, lld places the R PT_LOAD first. -Ttext-segment
   // is an old-fashioned option that does not play well with lld's layout.
   // Suggest --image-base as a likely alternative.
   if (args.hasArg(OPT_Ttext_segment))
     error("-Ttext-segment is not supported. Use --image-base if you "
           "intend to set the base address");
 
   // Parse ELF{32,64}{LE,BE} and CPU type.
-  if (auto *arg = args.getLastArg(OPT_m)) {
-    StringRef s = arg->getValue();
+  auto emulation = args.getLastArgValue(OPT_m, getenv("LDEMULATION"));
+  if (!emulation.empty()) {
     std::tie(config->ekind, config->emachine, config->osabi) =
-        parseEmulation(s);
-    config->mipsN32Abi =
-        (s.startswith("elf32btsmipn32") || s.startswith("elf32ltsmipn32"));
-    config->emulation = s;
+        parseEmulation(emulation);
+    config->mipsN32Abi = (emulation.startswith("elf32btsmipn32") ||
+                          emulation.startswith("elf32ltsmipn32"));
+    config->emulation = emulation;
   }
 
   // Parse --hash-style={sysv,gnu,both}.
   if (auto *arg = args.getLastArg(OPT_hash_style)) {
     StringRef s = arg->getValue();
     if (s == "sysv")
       config->sysvHash = true;
     else if (s == "gnu")

lld/MinGW/Driver.cpp

bool mingw::link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
   }
 
   std::vector<std::string> linkArgs;
   auto add = [&](const Twine &s) { linkArgs.push_back(s.str()); };
 
   add("lld-link");
   add("-lldmingw");
 
+  auto emulation = args.getLastArgValue(OPT_m, getenv("LDEMULATION"));
+
   if (auto *a = args.getLastArg(OPT_entry)) {
     StringRef s = a->getValue();
-    if (args.getLastArgValue(OPT_m) == "i386pe" && s.startswith("_"))
+    if (emulation == "i386pe" && s.startswith("_"))
       add("-entry:" + s.substr(1));
     else
       add("-entry:" + s);
   }
 
   if (args.hasArg(OPT_major_os_version, OPT_minor_os_version,
                   OPT_major_subsystem_version, OPT_minor_subsystem_version)) {
     StringRef majOSVer = args.getLastArgValue(OPT_major_os_version, "6");
if (args.hasArg(OPT_large_address_aware))
     add("-largeaddressaware");
   if (args.hasArg(OPT_kill_at))
     add("-kill-at");
   if (args.hasArg(OPT_appcontainer))
     add("-appcontainer");
   if (args.hasFlag(OPT_no_seh, OPT_disable_no_seh, false))
     add("-noseh");
 
-  if (args.getLastArgValue(OPT_m) != "thumb2pe" &&
-      args.getLastArgValue(OPT_m) != "arm64pe" &&
+  if (emulation != "thumb2pe" && emulation != "arm64pe" &&
       args.hasFlag(OPT_disable_dynamicbase, OPT_dynamicbase, false))
     add("-dynamicbase:no");
   if (args.hasFlag(OPT_disable_high_entropy_va, OPT_high_entropy_va, false))
     add("-highentropyva:no");
   if (args.hasFlag(OPT_disable_nxcompat, OPT_nxcompat, false))
     add("-nxcompat:no");
   if (args.hasFlag(OPT_disable_tsaware, OPT_tsaware, false))
     add("-tsaware:no");
if (auto *a = args.getLastArg(OPT_icf)) {
     else if (s == "safe" || s == "none")
       add("-opt:noicf");
     else
       error("unknown parameter: --icf=" + s);
   } else {
     add("-opt:noicf");
   }
 
-  if (auto *a = args.getLastArg(OPT_m)) {
-    StringRef s = a->getValue();
-    if (s == "i386pe")
+  if (!emulation.empty()) {
+    if (emulation == "i386pe")
       add("-machine:x86");
-    else if (s == "i386pep")
+    else if (emulation == "i386pep")
       add("-machine:x64");
-    else if (s == "thumb2pe")
+    else if (emulation == "thumb2pe")
       add("-machine:arm");
-    else if (s == "arm64pe")
+    else if (emulation == "arm64pe")
       add("-machine:arm64");
     else
-      error("unknown parameter: -m" + s);
+      error("unknown parameter: -m" + emulation);
   }
 
   for (auto *a : args.filtered(OPT_mllvm))
     add("-mllvm:" + StringRef(a->getValue()));
 
   for (auto *a : args.filtered(OPT_Xlink))
     add(a->getValue());
 
-  if (args.getLastArgValue(OPT_m) == "i386pe")
+  if (emulation == "i386pe")
     add("-alternatename:__image_base__=___ImageBase");
   else
     add("-alternatename:__image_base__=__ImageBase");
 
   for (auto *a : args.filtered(OPT_require_defined))
     add("-include:" + StringRef(a->getValue()));
   for (auto *a : args.filtered(OPT_undefined))
     add("-includeoptional:" + StringRef(a->getValue()));

lld/test/ELF/driver.test

 # REQUIRES: x86
 
 # RUN: not ld.lld --unknown1 --unkn=own2 -m foo /no/such/file -lnosuchlib \
 # RUN:   2>&1 | FileCheck -check-prefix=UNKNOWN %s
+# RUN: not env LDEMULTION=foo ld.lld --unknown1 --unkn=own2 /no/such/file -lnosuchlib \
+# RUN:   2>&1 | FileCheck -check-prefix=UNKNOWN %s
 
 # UNKNOWN: unknown argument '--unknown1'
 # UNKNOWN: unknown argument '--unkn=own2'
 # UNKNOWN: unknown emulation: foo
 # UNKNOWN: cannot open /no/such/file
 # UNKNOWN: unable to find library -lnosuchlib
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t

lld/test/MinGW/driver.test

 APPCONTAINER: -appcontainer
 
 RUN: ld.lld -m i386pep --version 2>&1 | FileCheck -check-prefix=VERSION %s
 RUN: ld.lld -m i386pep -v 2>&1 | FileCheck -check-prefix=VERSION %s
 RUN: not ld.lld -m i386pep -v xyz 2>&1 | FileCheck -check-prefix=VERSION %s
 VERSION: LLD {{.*}} (compatible with GNU linkers)
 
 RUN: ld.lld -m i386pep --help 2>&1 | FileCheck -check-prefix=HELP %s
+RUN: env LDEMULATION=i386pep ld.lld --help 2>&1 | FileCheck -check-prefix=HELP %s
 HELP: USAGE:
 HELP: --enable-auto-import
 
 RUN: ld.lld -### -m i386pep foo.o -delayload user32.dll --delayload shell32.dll 2>&1 | FileCheck -check-prefix DELAYLOAD %s
 RUN: ld.lld -### -m i386pep foo.o -delayload=user32.dll --delayload=shell32.dll 2>&1 | FileCheck -check-prefix DELAYLOAD %s
 DELAYLOAD: -delayload:user32.dll -delayload:shell32.dll
 
 RUN: not ld.lld -m i386pep -entry 2>&1 | FileCheck -check-prefix MISSING_ARG %s

lld/tools/lld/lld.cpp

static bool isPETarget(std::vector<const char *> &v) {
   StringSaver saver(a);
   cl::ExpandResponseFiles(saver, getDefaultQuotingStyle(), expandedArgs);
   for (auto it = expandedArgs.begin(); it + 1 != expandedArgs.end(); ++it) {
     if (StringRef(*it) != "-m")
       continue;
     return isPETargetName(*(it + 1));
   }
 
+  StringRef emulationEnv = getenv("LDEMULATION");
+  if (!emulationEnv.empty()) {
+    return isPETargetName(emulationEnv);
+  }
+
 #ifdef LLD_DEFAULT_LD_LLD_IS_MINGW
   return true;
 #else
   return false;
 #endif
 }
 
 static Flavor parseProgname(StringRef progname) {