[PATCH] Don't force the use of a base pointer with MS inline asm

[Reid Kleckner]

Ping.


On Fri, Aug 16, 2013 at 3:08 PM, Reid Kleckner <rnk at google.com> wrote:

> Jakob, can you take a look at this?
>
> Unfortunately, the fatal error I added actually comes up far more often
> than I expected because LLVM will try to align stack frames for code like:
>
> void foo() {
>   struct { int x; int y; } p;  // LLVM wants this to be 8 byte aligned.
> Why?
>   __asm { ... adjust esp and use p }
> }
>
> I'd prefer it if LLVM failed instead of miscompiling this code, though.
>
> I wasn't able to do a complete fix to pick the base register dynamically
> because I can't figure out how to pull the constraints off inline asm in MI.
>
> I would also reject gcc inline asm that clobbers esp and in the presence
> of stack realignment if I could see the constraints.
>
>
> On Tue, Aug 13, 2013 at 6:09 PM, Reid Kleckner <rnk at google.com> wrote:
>
>>     - Add a fatal error.
>>
>> http://llvm-reviews.chandlerc.com/D1317
>>
>> CHANGE SINCE LAST DIFF
>>   http://llvm-reviews.chandlerc.com/D1317?vs=3269&id=3460#toc
>>
>> Files:
>>   lib/Target/X86/X86RegisterInfo.cpp
>>   test/CodeGen/X86/inline-asm-stack-realign.ll
>>   test/CodeGen/X86/ms-inline-asm.ll
>>
>> Index: lib/Target/X86/X86RegisterInfo.cpp
>> ===================================================================
>> --- lib/Target/X86/X86RegisterInfo.cpp
>> +++ lib/Target/X86/X86RegisterInfo.cpp
>> @@ -340,6 +340,11 @@
>>          "Stack realignment in presence of dynamic allocas is not
>> supported with"
>>          "this calling convention.");
>>
>> +    if (MF.hasMSInlineAsm())
>> +      report_fatal_error(
>> +        "Stack realignment in the presence of stack-adjusting inline
>> assembly "
>> +        "is not supported");
>> +
>>      for (MCSubRegIterator I(getBaseRegister(), this,
>> /*IncludeSelf=*/true);
>>           I.isValid(); ++I)
>>        Reserved.set(*I);
>> @@ -396,18 +401,16 @@
>>     if (!EnableBasePointer)
>>       return false;
>>
>> -   // When we need stack realignment and there are dynamic allocas, we
>> can't
>> -   // reference off of the stack pointer, so we reserve a base pointer.
>> -   //
>> -   // This is also true if the function contain MS-style inline
>> assembly.  We
>> -   // do this because if any stack changes occur in the inline assembly,
>> e.g.,
>> -   // "pusha", then any C local variable or C argument references in the
>> -   // inline assembly will be wrong because the SP is not properly
>> tracked.
>> -   if ((needsStackRealignment(MF) && MFI->hasVarSizedObjects()) ||
>> -       MF.hasMSInlineAsm())
>> -     return true;
>> -
>> -   return false;
>> +   // When we need stack realignment, we can't address the stack from
>> the frame
>> +   // pointer.  When we have dynamic allocas or MS inline asm, we can't
>> address
>> +   // variables from the stack pointer.  MS inline asm can reference
>> locals
>> +   // while also adjusting the stack pointer.  When we can't use both
>> the SP and
>> +   // the FP, we need a separate base pointer register.
>> +   // FIXME: gcc inline asm can also adjust the stack pointer if it
>> lists esp as
>> +   // a clobber, but LLVM does not support that.
>> +   bool CantUseFP = needsStackRealignment(MF);
>> +   bool CantUseSP = MFI->hasVarSizedObjects() || MF.hasMSInlineAsm();
>> +   return CantUseFP && CantUseSP;
>>  }
>>
>>  bool X86RegisterInfo::canRealignStack(const MachineFunction &MF) const {
>> Index: test/CodeGen/X86/inline-asm-stack-realign.ll
>> ===================================================================
>> --- /dev/null
>> +++ test/CodeGen/X86/inline-asm-stack-realign.ll
>> @@ -0,0 +1,16 @@
>> +; RUN: not llc -march x86 %s 2>&1 | FileCheck %s
>> +
>> +; Realigning the stack and referring to local stack vars through memory
>> in
>> +; inline asm with this many clobbers is impossible.
>> +; FIXME: LLVM could do better here by looking at the list of clobbers.
>> +
>> +; CHECK: Stack realignment in presence of MS inline asm is not supported
>> +
>> +define i32 @foo() {
>> +entry:
>> +  %r = alloca i32, align 16
>> +  store i32 -1, i32* %r, align 16
>> +  call void asm sideeffect inteldialect "pusha\0A\09xor eax,
>> eax\0A\09xor ebx, ebx\0A\09xor ecx, ecx\0A\09xor edx, edx\0A\09xor esi,
>> esi\0A\09xor edi, edi\0A\09mov dword ptr $0, esi\0A\09popa",
>> "=*m,~{eax},~{ebx},~{ecx},~{edi},~{edx},~{esi},~{dirflag},~{fpsr},~{flags}"(i32*
>> %r) #1
>> +  %0 = load i32* %r, align 16
>> +  ret i32 %0
>> +}
>> Index: test/CodeGen/X86/ms-inline-asm.ll
>> ===================================================================
>> --- test/CodeGen/X86/ms-inline-asm.ll
>> +++ test/CodeGen/X86/ms-inline-asm.ll
>> @@ -103,8 +103,8 @@
>>  ; CHECK: {{## InlineAsm End|#NO_APP}}
>>  ; CHECK: {{## InlineAsm Start|#APP}}
>>  ; CHECK: .intel_syntax
>> -; CHECK: mov dword ptr [esi], edi
>> +; CHECK: mov dword ptr [ebp - 8], edi
>>  ; CHECK: .att_syntax
>>  ; CHECK: {{## InlineAsm End|#NO_APP}}
>> -; CHECK: movl (%esi), %eax
>> +; CHECK: movl -8(%ebp), %eax
>>  }
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130822/22812bb4/attachment.html>