Add Position-independent Code model Module API.

[jhibbits]

Summary

This makes PIC levels a Module flag attribute, which can be queried by the
backend. The flag is named PIC Level, and can have a value of:

0 - Backend-default
1 - Small-model (-fpic)
2 - Large-model (-fPIC)

These match the `-pic-level' command line argument for clang, and the value of the
preprocessor macro `PIC'.

[• rafael]

This should probably include a test showing what llvm-link does when merging these.

tools/llc/llc.cpp

290 ↗

(On Diff #15189)

Is it intentional that any pre-existing metadata is always discarded?

[jhibbits]

In D5882#5, @rafael wrote:

This should probably include a test showing what llvm-link does when merging these.

Thanks for the review. I'm updating the patch now, to also fix bugs I found (such as an error being thrown if I specify the command line, as well as the module flag, even if I change it to ModFlagBehavior::Override)

tools/llc/llc.cpp

290 ↗

(On Diff #15189)

The intent is to allow you to override the module flag with the command line (-pic-level=small/large). So, really, this should have a check for if PLLevel != PICLevel::Default.

[jhibbits]

Update the patch to address Rafael's comments. Add some tests.

[• rafael]

test/Linker/module-flags-pic-1-a.ll

1

This test has no CHECK line. You also need to pass %s to FileCheck

test/Linker/module-flags-pic-1-b.ll

1

Please put this in test/Linker/Inputs. That way you don't need the dummy RUN line.

test/Linker/module-flags-pic-2-a.ll

2

Why XFAIL?

[jhibbits]

I'll upload a new diff this evening. Thanks!

test/Linker/module-flags-pic-1-a.ll

1

Oops, wonder how this even passed the test run.

test/Linker/module-flags-pic-1-b.ll

1

That definitely will make it cleaner.

test/Linker/module-flags-pic-2-a.ll

2

This test is intended to fail (test merging two different values for !flag_pic). Is there a better way to do this?

[• rafael]

---

rafael wrote:

Why XFAIL?

This test is intended to fail (test merging two different values for !flag_pic). Is there a better way to do this?

Yes, use the not utility (RUN: not llvm-link...). It is probably a
good idea to FileCheck for whatever error message we print.

Cheers,
Rafael

[jhibbits]

Address Rafael's comments, updating the tests.

[• rafael]

I think this is fine. We can always change the error to an merge logic in the future if needed. Eric, what do you think?

test/Linker/module-flags-pic-2-a.ll

11

I think I fixed the "EROR: WARNING" in another patch. This should be just ERROR now.

[jhibbits]

Ping?

[echristo]

One comment inline.

lib/IR/Module.cpp

472

One bike shed: "PIC Level" perhaps? since it's not just -fpic, but also -fPIC?

[jhibbits]

lib/IR/Module.cpp

472

Works for me.

[jhibbits]

Rename 'flag_pic' to 'PIC Level'

[• rafael]

LGTM

[echristo]

LGTM.

[jhibbits]

Closed by commit rL221510 (authored by @jhibbits).

include/llvm/IR/Module.h

 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/Support/CBindingWrapping.h"
+#include "llvm/Support/CodeGen.h"
 #include "llvm/Support/DataTypes.h"
 #include <system_error>
 

   unsigned getDwarfVersion() const;
 
 /// @}
+/// @name Utility functions for querying and setting PIC level
+/// @{
+
+  /// \brief Returns the PIC level (small or large model)
+  PICLevel::Level getPICLevel() const;
+
+  /// \brief Set the PIC level (small or large model)
+  void setPICLevel(PICLevel::Level PL);
 };
 
 /// An raw_ostream inserter for modules.

include/llvm/Support/CodeGen.h

     enum Model { Default, JITDefault, Small, Kernel, Medium, Large };
   }
 
+  namespace PICLevel {
+    enum Level { Default=0, Small=1, Large=2 };
+  }
+
   // TLS models.
   namespace TLSModel {
     enum Model {

lib/IR/Module.cpp

   Entry.second.Name = &Entry;
   return &Entry.second;
 }
+
+PICLevel::Level Module::getPICLevel() const {
+  Value *Val = getModuleFlag("flag_pic");
+
+  if (Val == NULL)
+    return PICLevel::Default;
+
+  return static_cast<PICLevel::Level>(cast<ConstantInt>(Val)->getZExtValue());
+}
+
+void Module::setPICLevel(PICLevel::Level PL) {
+  addModuleFlag(ModFlagBehavior::Error, "flag_pic", PL);
+}

test/Linker/module-flags-pic-1-a.ll

+; RUN: llvm-link %s %p/module-flags-pic-1-b.ll -S -o - | FileCheck
+
+; test linking modules with specified and default PIC levels
+
+!0 = metadata !{ i32 1, metadata !"flag_pic", i32 1 }
+
+!llvm.module.flags = !{!0}

test/Linker/module-flags-pic-1-b.ll

+; RUN: true

test/Linker/module-flags-pic-2-a.ll

+; RUN: llvm-link %s %p/module-flags-pic-2-b.ll -S -o - | FileCheck
+; XFAIL: *
+
+; test linking modules with two different PIC levels
+
+!0 = metadata !{ i32 1, metadata !"flag_pic", i32 1 }
+
+!llvm.module.flags = !{!0}

test/Linker/module-flags-pic-2-b.ll

+; RUN: true
+
+!0 = metadata !{ i32 1, metadata !"flag_pic", i32 2 }
+
+!llvm.module.flags = !{!0}