[LLD] [MinGW] Don't pass -dynamicbase:no by default for arm/arm64

[mstorsjo]

Summary

The linker refuses (since D41051) using -dynamicbase:no on these architectures.

Stop passing -dynamicbase (which just reinforces the lld-link default) for simplicity.

[ruiu]

MinGW/Driver.cpp

162–169

If a user passes -dynamicbase to the MinGW driver, we should just pass it through to the COFF driver, no? Ignoring -dynamicbase if arm/arm64 seems odd to me.

[mstorsjo]

MinGW/Driver.cpp

162–169

I guess we can pass it through as well -
will update.

[mstorsjo]

Updated to pass the flag through on these architectures.

[ruiu]

MinGW/Driver.cpp

162–165

My understanding is passing -dynamicbase:no on arm/arm64 doesn't make sense and should be considered error. Don't you want to remove this special condition and handle all targets equally (so that the error would be reported if you pass -dynamicbase:no on arm/arm64)?

[mstorsjo]

MinGW/Driver.cpp

162–165

In the MinGW/GNU frontend, there is no -dynamicbase:no case. We have different behaviours in the COFF and MinGW frontends; in the COFF one we default to dynamicbase being enabled and it can be disabled with the link.exe option -dynamicbase:no.

In the MinGW frontend on the other hand, the default is off and it can be enabled with --dynamicbase. There is no option for disabling it (which in GNU options style would be --no-dynamicbase) AFAIK. Thus in the MinGW frontend, we pass -dynamicbase:no by default to the COFF linker, to match the behaviour of GNU ld. But for arm/arm64 (which GNU ld doesn't support at all for COFF), we obviously want to do things differently. Thus in this form, we either rely on the COFF linker default (on) implicitly if no flag is provided, or explicitly request it if the flag is set.

[mstorsjo]

Ping @ruiu - any followup to this one?

[ruiu]

MinGW/Driver.cpp

162–165

Thank you for the explanation. So overall it sounds like it would suffice if we pass -dynamicbase:no if !arm && !Args.hasArg(OPT_dynamicbase). We don't need to pass -dynamicbase because it is default. Is this correct?

[mstorsjo]

MinGW/Driver.cpp

162–165

Correct - that would suffice as well. That adds a minimal dependency on the lld default value, but I don't see that changing towards false by default anyway.

[ruiu]

LGTM

MinGW/Driver.cpp

if (Args.hasArg(OPT_verbose))
     Add("-verbose");
   if (Args.hasArg(OPT_export_all_symbols))
     Add("-export-all-symbols");
   if (!Args.hasArg(OPT_strip_all))
     Add("-debug:dwarf");
   if (Args.hasArg(OPT_large_address_aware))
     Add("-largeaddressaware");
 
+  if (Args.getLastArgValue(OPT_m) == "thumb2pe" ||
+      Args.getLastArgValue(OPT_m) == "arm64pe") {
+    if (Args.hasArg(OPT_dynamicbase))
+      Add("-dynamicbase");
+  } else {
     Add(Args.hasArg(OPT_dynamicbase) ? "-dynamicbase" : "-dynamicbase:no");
+  }
 
   if (Args.hasFlag(OPT_gc_sections, OPT_no_gc_sections, false))
     Add("-opt:ref");
   else
     Add("-opt:noref");
 
   if (auto *A = Args.getLastArg(OPT_icf)) {
     StringRef S = A->getValue();
     if (S == "all")

test/MinGW/driver.test

 RUN: ld.lld -### -m i386pep foo.o --strip-all | FileCheck -check-prefix STRIP %s
 STRIP-NOT: -debug:dwarf
 
 RUN: ld.lld -### -m i386pep foo.o --large-address-aware | FileCheck -check-prefix LARGE-ADDRESS-AWARE %s
 LARGE-ADDRESS-AWARE: -largeaddressaware
 
 RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix DEFAULT-DISABLE-FLAGS %s
 DEFAULT-DISABLE-FLAGS: -dynamicbase:no
+RUN: ld.lld -### -m thumb2pe foo.o | FileCheck -check-prefix NO-DEFAULT-DISABLE-FLAGS %s
+RUN: ld.lld -### -m arm64pe foo.o | FileCheck -check-prefix NO-DEFAULT-DISABLE-FLAGS %s
+NO-DEFAULT-DISABLE-FLAGS-NOT: -dynamicbase:no
 
 # The extra '-' in the following check pattern is intentional, to make sure
 # that a check for "-dynamicbase" doesn't match the default "-dynamicbase:no".
 RUN: ld.lld -### -m i386pep foo.o --dynamicbase | FileCheck -check-prefix DYNAMICBASE %s
 RUN: ld.lld -### -m i386pep foo.o -dynamicbase | FileCheck -check-prefix DYNAMICBASE %s
+RUN: ld.lld -### -m thumb2pe foo.o --dynamicbase | FileCheck -check-prefix DYNAMICBASE %s
+RUN: ld.lld -### -m arm64pe foo.o --dynamicbase | FileCheck -check-prefix DYNAMICBASE %s
 DYNAMICBASE: -dynamicbase -
 
 RUN: ld.lld -### -m i386pep foo.o --image-base 0x1230000 | FileCheck -check-prefix IMAGE-BASE %s
 RUN: ld.lld -### -m i386pep foo.o -image-base 0x1230000 | FileCheck -check-prefix IMAGE-BASE %s
 IMAGE-BASE: -base:0x1230000
 
 RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix NO-GC-SECTIONS %s
 RUN: ld.lld -### -m i386pep foo.o --gc-sections --no-gc-sections | FileCheck -check-prefix NO-GC-SECTIONS %s