Disable GC and ICF when /debug is present

ICF and GC impair debugging, so MSVC disables these optimizations when
/debug is passed. They are still on by default when no PDB is produced.

This change also makes /opt:ref enable ICF, which is consistent with
MSVC: https://msdn.microsoft.com/en-us/library/bxwfs976.aspx

We should consider making /opt:icf fold readonly data in the near
future. LLD used to do this, but we disabled it because it breaks too
many programs. MSVC only does this if the user explicitly passes
/opt:icf.

Reviewers: ruiu, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39885

llvm-svn: 318071

lld/COFF/Driver.cpp

@@ -896,50 +896,51 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   if (auto *Arg = Args.getLastArg(OPT_implib))
     Config->Implib = Arg->getValue();
 
-  // Handle /opt
+  // Handle /opt.
+  bool DoGC = !Args.hasArg(OPT_debug);
+  unsigned ICFLevel = 1; // 0: off, 1: limited, 2: on
   for (auto *Arg : Args.filtered(OPT_opt)) {
     std::string Str = StringRef(Arg->getValue()).lower();
     SmallVector<StringRef, 1> Vec;
     StringRef(Str).split(Vec, ',');
     for (StringRef S : Vec) {
-      if (S == "noref") {
-        Config->DoGC = false;
-        Config->DoICF = false;
-        continue;
-      }
-      if (S == "icf" || S.startswith("icf=")) {
-        Config->DoICF = true;
-        continue;
-      }
-      if (S == "noicf") {
-        Config->DoICF = false;
-        continue;
-      }
-      if (S.startswith("lldlto=")) {
+      if (S == "ref") {
+        DoGC = true;
+      } else if (S == "noref") {
+        DoGC = false;
+      } else if (S == "icf" || S.startswith("icf=")) {
+        ICFLevel = 2;
+      } else if (S == "noicf") {
+        ICFLevel = 0;
+      } else if (S.startswith("lldlto=")) {
         StringRef OptLevel = S.substr(7);
         if (OptLevel.getAsInteger(10, Config->LTOOptLevel) ||
             Config->LTOOptLevel > 3)
           error("/opt:lldlto: invalid optimization level: " + OptLevel);
-        continue;
-      }
-      if (S.startswith("lldltojobs=")) {
+      } else if (S.startswith("lldltojobs=")) {
         StringRef Jobs = S.substr(11);
         if (Jobs.getAsInteger(10, Config->LTOJobs) || Config->LTOJobs == 0)
           error("/opt:lldltojobs: invalid job count: " + Jobs);
-        continue;
-      }
-      if (S.startswith("lldltopartitions=")) {
+      } else if (S.startswith("lldltopartitions=")) {
         StringRef N = S.substr(17);
         if (N.getAsInteger(10, Config->LTOPartitions) ||
             Config->LTOPartitions == 0)
           error("/opt:lldltopartitions: invalid partition count: " + N);
-        continue;
-      }
-      if (S != "ref" && S != "lbr" && S != "nolbr")
+      } else if (S != "lbr" && S != "nolbr")
         error("/opt: unknown option: " + S);
     }
   }
 
+  // Limited ICF is enabled if GC is enabled and ICF was never mentioned
+  // explicitly.
+  // FIXME: LLD only implements "limited" ICF, i.e. it only merges identical
+  // code. If the user passes /OPT:ICF explicitly, LLD should merge identical
+  // comdat readonly data.
+  if (ICFLevel == 1 && !DoGC)
+    ICFLevel = 0;
+  Config->DoGC = DoGC;
+  Config->DoICF = ICFLevel > 0;
+
   // Handle /lldsavetemps
   if (Args.hasArg(OPT_lldsavetemps))
     Config->SaveTemps = true;

lld/test/COFF/icf-associative.test

@@ -1,5 +1,5 @@
 # RUN: yaml2obj < %s > %t.obj
-# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console /include:bar \
+# RUN: lld-link /opt:icf /entry:foo /out:%t.exe /subsystem:console /include:bar \
 # RUN:   /debug /verbose %t.obj > %t.log 2>&1
 # RUN: FileCheck %s < %t.log
 

lld/test/COFF/icf-simple.test

@@ -1,5 +1,5 @@
 # RUN: yaml2obj < %s > %t.obj
-# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console /include:bar \
+# RUN: lld-link /opt:icf /entry:foo /out:%t.exe /subsystem:console /include:bar \
 # RUN:   /verbose %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=ICF %s < %t.log
 
@@ -13,6 +13,31 @@
 # RUN:   /verbose /opt:noref,noicf %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=NOICF %s < %t.log
 
+# ICF is on by default (no /opt: flags).
+# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=ICF %s < %t.log
+
+# /debug disables ICF.
+# RUN: lld-link /debug /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
+# /opt:noref disables ICF.
+# RUN: lld-link /opt:noref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
+# /debug /opt:ref enables ICF.
+# RUN: lld-link /debug /opt:ref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=ICF %s < %t.log
+
+# /debug /opt:noicf,ref disables ICF.
+# RUN: lld-link /debug /opt:noicf,ref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
 # NOICF-NOT: Removed foo
 # NOICF-NOT: Removed bar
 

lld/test/COFF/pdb-global-gc.yaml

@@ -1,7 +1,7 @@
 # RUN: yaml2obj %s -o %t.obj
 # RUN: llvm-mc %S/Inputs/pdb-global-gc.s -triple x86_64-windows-msvc -filetype=obj -o %t2.obj
 # RUN: lld-link %t.obj %t2.obj -debug -entry:main \
-# RUN:          -nodefaultlib -debug -out:%t.exe -pdb:%t.pdb -verbose
+# RUN:          -nodefaultlib -opt:ref -out:%t.exe -pdb:%t.pdb -verbose
 # RUN: llvm-pdbutil dump -symbols -globals %t.pdb | FileCheck %s
 
 # This tests the case where an __imp_ chunk is discarded by linker GC. The debug

lld/test/COFF/pdb-import-gc.yaml

@@ -1,6 +1,6 @@
 # RUN: yaml2obj %s -o %t.obj
 # RUN: lld-link %t.obj %S/Inputs/pdb-import-gc.lib -debug -entry:main \
-# RUN:          -nodefaultlib -debug -out:%t.exe -pdb:%t.pdb
+# RUN:          -nodefaultlib -opt:ref -out:%t.exe -pdb:%t.pdb
 # RUN: llvm-pdbutil dump -globals -symbols %t.pdb | FileCheck %s
 
 # This tests the case where an __imp_ chunk is discarded by linker GC. The debug