[X86] Permit reading of the FLAGS register without it being previously defined

We modeled the RDFLAGS{32,64} operations as "using" {E,R}FLAGS.
While technically correct, this is not be desirable for folks who want
to examine aspects of the FLAGS register which are not related to
computation like whether or not CPUID is a valid instruction.

Differential Revision: http://reviews.llvm.org/D17782

llvm-svn: 262465

2 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 59c0f199150..23c8a667c0b 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -23255,7 +23255,12 @@ X86TargetLowering::EmitInstrWithCustomInserter(MachineInstr *MI,
         MI->getOpcode() == X86::RDFLAGS32 ? X86::PUSHF32 : X86::PUSHF64;
     unsigned Pop =
         MI->getOpcode() == X86::RDFLAGS32 ? X86::POP32r : X86::POP64r;
-    BuildMI(*BB, MI, DL, TII->get(PushF));
+    MachineInstr *Push = BuildMI(*BB, MI, DL, TII->get(PushF));
+    // Permit reads of the FLAGS register without it being defined.
+    // This intrinsic exists to read external processor state in flags, such as
+    // the trap flag, interrupt flag, and direction flag, none of which are
+    // modeled by the backend.
+    Push->getOperand(2).setIsUndef();
     BuildMI(*BB, MI, DL, TII->get(Pop), MI->getOperand(0).getReg());
 
     MI->eraseFromParent(); // The pseudo is gone now.
diff --git a/llvm/lib/Target/X86/X86InstrInfo.td b/llvm/lib/Target/X86/X86InstrInfo.td
index 14956fcff09..70106251bbe 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.td
+++ b/llvm/lib/Target/X86/X86InstrInfo.td
@@ -1122,12 +1122,12 @@ def PUSH32rmm: I<0xFF, MRM6m, (outs), (ins i32mem:$src), "push{l}\t$src",[],
 
 let mayLoad = 1, mayStore = 1, usesCustomInserter = 1,
     SchedRW = [WriteRMW], Defs = [ESP] in {
-  let Uses = [ESP, EFLAGS] in
+  let Uses = [ESP] in
   def RDFLAGS32 : PseudoI<(outs GR32:$dst), (ins),
                    [(set GR32:$dst, (int_x86_flags_read_u32))]>,
                 Requires<[Not64BitMode]>;
 
-  let Uses = [RSP, EFLAGS] in
+  let Uses = [RSP] in
   def RDFLAGS64 : PseudoI<(outs GR64:$dst), (ins),
                    [(set GR64:$dst, (int_x86_flags_read_u64))]>,
                 Requires<[In64BitMode]>;