[x86] Switch EFLAGS copy lowering to use reg-reg form of testing for

a zero register.

Previously I tried this and saw LLVM unable to transform this to fold
with memory operands such as spill slot rematerialization. However, it
clearly works as shown in this patch. We turn these into `cmpb $0,
<mem>` when useful for folding a memory operand without issue. This form
has no disadvantage compared to `testb $-1, <mem>`. So overall, this is
likely no worse and may be slightly smaller in some cases due to the
`testb %reg, %reg` form.

Differential Revision: https://reviews.llvm.org/D45475

llvm-svn: 330269

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp b/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
index d06388f5daf..fea5ce68766 100644
--- a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
+++ b/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
@@ -636,7 +636,7 @@ void X86FlagsCopyLoweringPass::insertTest(MachineBasicBlock &MBB,
   // also allow us to select a shorter encoding of `testb %reg, %reg` when that
   // would be equivalent.
   auto TestI =
-      BuildMI(MBB, Pos, Loc, TII->get(X86::TEST8ri)).addReg(Reg).addImm(-1);
+      BuildMI(MBB, Pos, Loc, TII->get(X86::TEST8rr)).addReg(Reg).addReg(Reg);
   (void)TestI;
   DEBUG(dbgs() << "    test cond: "; TestI->dump());
   ++NumTestsInserted;