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

[Reid Kleckner]

This way we only use a base pointer if we need to realign the stack.
Otherwise we use the frame pointer.  We still have a bug in that case if
the inline asm clobbers esi.

Partial fix for PR16830.

http://llvm-reviews.chandlerc.com/D1317

Files:
  lib/Target/X86/X86RegisterInfo.cpp
  test/CodeGen/X86/ms-inline-asm.ll

Index: lib/Target/X86/X86RegisterInfo.cpp
===================================================================
--- lib/Target/X86/X86RegisterInfo.cpp
+++ lib/Target/X86/X86RegisterInfo.cpp
@@ -396,18 +396,14 @@
    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.
+   bool CantUseFP = needsStackRealignment(MF);
+   bool CantUseSP = MFI->hasVarSizedObjects() || MF.hasMSInlineAsm();
+   return CantUseFP && CantUseSP;
 }
 
 bool X86RegisterInfo::canRealignStack(const MachineFunction &MF) const {
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
 }
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