[llvm-exegesis][X86] Save and restore eflags.

This is needed to benchmark instruction that touch EFLAGS (e.g. STD: set direction flag).

Differential Revision: https://reviews.llvm.org/D90742

llvm/test/tools/llvm-exegesis/X86/uops-STD.s

@@ -0,0 +1,7 @@
+# RUN: llvm-exegesis -mode=uops -opcode-name=STD -repetition-mode=duplicate | FileCheck %s
+# RUN: llvm-exegesis -mode=uops -opcode-name=STD -repetition-mode=loop | FileCheck %s
+
+CHECK:      mode:            uops
+CHECK-NEXT: key:
+CHECK-NEXT:   instructions:
+CHECK-NEXT:     STD

llvm/tools/llvm-exegesis/lib/X86/Target.cpp

@@ -603,8 +603,10 @@ class X86SavedState : public ExegesisTarget::SavedState {
 #ifdef __x86_64__
 # if defined(_MSC_VER)
     _fxsave64(FPState);
+    Eflags = __readeflags();
 # elif defined(__GNUC__)
     __builtin_ia32_fxsave64(FPState);
+    Eflags = __builtin_ia32_readeflags_u64();
 # endif
 #else
     llvm_unreachable("X86 exegesis running on non-X86 target");
@@ -618,9 +620,11 @@ class X86SavedState : public ExegesisTarget::SavedState {
 # if defined(_MSC_VER)
     _clearfp();
     _fxrstor64(FPState);
+    __writeeflags(Eflags);
 # elif defined(__GNUC__)
     asm volatile("fwait");
     __builtin_ia32_fxrstor64(FPState);
+    __builtin_ia32_writeeflags_u64(Eflags);
 # endif
 #else
     llvm_unreachable("X86 exegesis running on non-X86 target");
@@ -630,6 +634,7 @@ class X86SavedState : public ExegesisTarget::SavedState {
 private:
 #ifdef __x86_64__
   alignas(16) char FPState[512];
+  uint64_t Eflags;
 #endif
 };