[llvm] r317292 - Avoid PLT for external calls when attribute nonlazybind is used.

[Sriraman Tallam via llvm-commits]

On Thu, Nov 2, 2017 at 5:49 PM, Sriraman Tallam <tmsriram at google.com> wrote:
> On Thu, Nov 2, 2017 at 5:43 PM, Rafael Avila de Espindola
> <rafael.espindola at gmail.com> wrote:
>> Why force a call via the got when a direct one could be used?
>>
>> With -fno-plt -fPIC gcc compiles
>>
>> __attribute__((visibility("hidden"))) void foo(void);
>> void bar(void);
>> void zed(void) {
>>   foo();
>>   bar();
>> }
>>
>> to
>>
>>         call    foo
>>         call    *bar at GOTPCREL(%rip)
>>
>> But this patch will use the got for both calls, no?

You are right. I just checked this and foo has to be defined so I
should not touch this call.  I need to fix this bug.

>
> I did not test this case and if it is true that foo can never be
> external in this case then I got it wrong.  But what if foo was not
> defined by the DSO?
>
>>
>> I think the idea of -fno-plt is to use a got when a plt would have been
>> used. If a plt was not going to be used, the option has no effect.
>>
>> Cheers,
>> Rafael
>>
>> Sriraman Tallam via llvm-commits <llvm-commits at lists.llvm.org> writes:
>>
>>> Author: tmsriram
>>> Date: Thu Nov  2 17:10:19 2017
>>> New Revision: 317292
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=317292&view=rev
>>> Log:
>>> Avoid PLT for external calls when attribute nonlazybind is used.
>>>
>>> Differential Revision: https://reviews.llvm.org/D39065
>>>
>>> Added:
>>>     llvm/trunk/test/CodeGen/X86/no-plt.ll
>>> Modified:
>>>     llvm/trunk/lib/Target/X86/X86Subtarget.cpp
>>>
>>> Modified: llvm/trunk/lib/Target/X86/X86Subtarget.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86Subtarget.cpp?rev=317292&r1=317291&r2=317292&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/lib/Target/X86/X86Subtarget.cpp (original)
>>> +++ llvm/trunk/lib/Target/X86/X86Subtarget.cpp Thu Nov  2 17:10:19 2017
>>> @@ -144,6 +144,15 @@ X86Subtarget::classifyGlobalFunctionRefe
>>>  unsigned char
>>>  X86Subtarget::classifyGlobalFunctionReference(const GlobalValue *GV,
>>>                                                const Module &M) const {
>>> +  const Function *F = dyn_cast_or_null<Function>(GV);
>>> +
>>> +  // Do not use the PLT when explicitly told to do so for ELF 64-bit
>>> +  // target.
>>> +  if (isTargetELF() && is64Bit() && F &&
>>> +      F->hasFnAttribute(Attribute::NonLazyBind) &&
>>> +      GV->isDeclarationForLinker())
>>> +    return X86II::MO_GOTPCREL;
>>> +
>>>    if (TM.shouldAssumeDSOLocal(M, GV))
>>>      return X86II::MO_NO_FLAG;
>>>
>>> @@ -153,8 +162,6 @@ X86Subtarget::classifyGlobalFunctionRefe
>>>      return X86II::MO_DLLIMPORT;
>>>    }
>>>
>>> -  const Function *F = dyn_cast_or_null<Function>(GV);
>>> -
>>>    if (isTargetELF()) {
>>>      if (is64Bit() && F && (CallingConv::X86_RegCall == F->getCallingConv()))
>>>        // According to psABI, PLT stub clobbers XMM8-XMM15.
>>>
>>> Added: llvm/trunk/test/CodeGen/X86/no-plt.ll
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/no-plt.ll?rev=317292&view=auto
>>> ==============================================================================
>>> --- llvm/trunk/test/CodeGen/X86/no-plt.ll (added)
>>> +++ llvm/trunk/test/CodeGen/X86/no-plt.ll Thu Nov  2 17:10:19 2017
>>> @@ -0,0 +1,23 @@
>>> +; RUN: llc < %s -mcpu=generic -mtriple=x86_64-linux-gnu -relocation-model=pic \
>>> +; RUN:   | FileCheck -check-prefix=X64 %s
>>> +; RUN: llc < %s -mcpu=generic -mtriple=x86_64-linux-gnu \
>>> +; RUN:   | FileCheck -check-prefix=X64 %s
>>> +
>>> +define i32 @main() #0 {
>>> +; X64: callq *_Z3foov at GOTPCREL(%rip)
>>> +; X64: callq _Z3barv
>>> +
>>> +entry:
>>> +  %retval = alloca i32, align 4
>>> +  store i32 0, i32* %retval, align 4
>>> +  %call1 = call i32 @_Z3foov()
>>> +  %call2 = call i32 @_Z3barv()
>>> +  ret i32 0
>>> +}
>>> +
>>> +; Function Attrs: nonlazybind
>>> +declare i32 @_Z3foov() #1
>>> +
>>> +declare i32 @_Z3barv() #2
>>> +
>>> +attributes #1 = { nonlazybind }
>>>
>>>
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