[llvm] r177426 - Remove an invalid and unnecessary Pat pattern from the X86 backend:

[Ulrich Weigand]

Author: uweigand
Date: Tue Mar 19 14:49:52 2013
New Revision: 177426

URL: http://llvm.org/viewvc/llvm-project?rev=177426&view=rev
Log:
Remove an invalid and unnecessary Pat pattern from the X86 backend:

  def : Pat<(load (i64 (X86Wrapper tglobaltlsaddr :$dst))),
            (MOV64rm tglobaltlsaddr :$dst)>;

This pattern is invalid because the MOV64rm instruction expects a
source operand of type "i64mem", which is a subclass of X86MemOperand
and thus actually consists of five MI operands, but the Pat provides
only a single MI operand ("tglobaltlsaddr" matches an SDnode of
type ISD::TargetGlobalTLSAddress and provides a single output).

Thus, if the pattern were ever matched, subsequent uses of the MOV64rm
instruction pattern would access uninitialized memory.  In addition,
with the TableGen patch I'm about to check in, this would actually be
reported as a build-time error.

Fortunately, the pattern does in fact never match, for at least two
independent reasons.

First, the code generator actually never generates a pattern of the
form (load (X86Wrapper (tglobaltlsaddr))).  For most combinations of
TLS and code models, (tglobaltlsaddr) represents just an offset that
needs to be added to some base register, so it is never directly
dereferenced.  The only exception is the initial-exec model, where
(tglobaltlsaddr) refers to the (pc-relative) address of a GOT slot,
which *is* in fact directly dereferenced: but in that case, the
X86WrapperRIP node is used, not X86Wrapper, so the Pat doesn't match.

Second, even if some patterns along those lines *were* ever generated,
we should not need an extra Pat pattern to match it.  Instead, the
original MOV64rm instruction pattern ought to match directly, since
it uses an "addr" operand, which is implemented via the SelectAddr
C++ routine; this routine is supposed to accept the full range of
input DAGs that may be implemented by a single mov instruction,
including those cases involving ISD::TargetGlobalTLSAddress (and
actually does so e.g. in the initial-exec case as above).

To avoid build breaks (due to the above-mentioned error) after the
TableGen patch is checked in, I'm removing this Pat here.


Modified:
    llvm/trunk/lib/Target/X86/X86InstrCompiler.td

Modified: llvm/trunk/lib/Target/X86/X86InstrCompiler.td
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86InstrCompiler.td?rev=177426&r1=177425&r2=177426&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86InstrCompiler.td (original)
+++ llvm/trunk/lib/Target/X86/X86InstrCompiler.td Tue Mar 19 14:49:52 2013
@@ -990,9 +990,6 @@ def : Pat<(i64 (X86Wrapper tglobaltlsadd
 // This corresponds to add $foo at tpoff, %rax
 def : Pat<(add GR64:$src1, (X86Wrapper tglobaltlsaddr :$dst)),
           (ADD64ri32 GR64:$src1, tglobaltlsaddr :$dst)>;
-// This corresponds to mov foo at tpoff(%rbx), %eax
-def : Pat<(load (i64 (X86Wrapper tglobaltlsaddr :$dst))),
-          (MOV64rm tglobaltlsaddr :$dst)>;
 
 
 // Direct PC relative function call for small code model. 32-bit displacement