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

[courbet]

Summary

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

[gchatelet]

Thx!

[thakis]

Looks like this doesn't build on windows: http://45.33.8.238/win/27220/step_4.txt

[courbet]

In D90742#2373438, @thakis wrote:

Looks like this doesn't build on windows: http://45.33.8.238/win/27220/step_4.txt

Thanks, I missed this bot. I'll send a fix.

[courbet]

In D90742#2373442, @courbet wrote:

In D90742#2373438, @thakis wrote:

Looks like this doesn't build on windows: http://45.33.8.238/win/27220/step_4.txt

Thanks, I missed this bot. I'll send a fix.

9287eb352e6c

[thakis]

Thanks!

[anna]

hi @courbet,
our internal buildbots at Azul has failed because of an older assembler. We’ve taken in all commits/reverts/relands w.r.t this change upto Nov 4th.

The specific failure is:
/tmp/cc2lBtEi.s: Assembler messages:
/tmp/cc2lBtEi.s:541: Error: no such instruction: `fxsave64 16(%rax)'
/tmp/cc2lBtEi.s:566: Error: no such instruction: `fxrstor64 16(%rdi)'
/tmp/cc2lBtEi.s:592: Error: no such instruction: `fxrstor64 16(%rdi)'
make[2]: *** [tools/llvm-exegesis/lib/X86/CMakeFiles/LLVMExegesisX86.dir/Target.cpp.o] Error 1

Any suggestions to fix this apart from an upgrade (which seems to be a very invasive change on our machines)?

[courbet]

In D90742#2389801, @anna wrote:

hi @courbet,
our internal buildbots at Azul has failed because of an older assembler. We’ve taken in all commits/reverts/relands w.r.t this change upto Nov 4th.

The specific failure is:
/tmp/cc2lBtEi.s: Assembler messages:
/tmp/cc2lBtEi.s:541: Error: no such instruction: `fxsave64 16(%rax)'
/tmp/cc2lBtEi.s:566: Error: no such instruction: `fxrstor64 16(%rdi)'
/tmp/cc2lBtEi.s:592: Error: no such instruction: `fxrstor64 16(%rdi)'
make[2]: *** [tools/llvm-exegesis/lib/X86/CMakeFiles/LLVMExegesisX86.dir/Target.cpp.o] Error 1

Any suggestions to fix this apart from an upgrade (which seems to be a very invasive change on our machines)?

Which assembler are you using ? Is is configured for 64-bit targets ?

[anna]

We use the GNU assembler and it is configured for 64 bit machines. Here's the details from one of the machines:

as --version
GNU assembler version 2.20.51.0.2-5.47.el6_9.1 20100205
Copyright 2009 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `x86_64-redhat-linux'

Also, note that I tagged this against the incorrect review. The change causing the failures is https://reviews.llvm.org/D90592. I'm going to paste this there and we can continue discussion there.

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

+# 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

 namespace {
 
 class X86SavedState : public ExegesisTarget::SavedState {
 public:
   X86SavedState() {
 #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");
 #endif
   }
 
   ~X86SavedState() {
     // Restoring the X87 state does not flush pending exceptions, make sure
     // these exceptions are flushed now.
 #ifdef __x86_64__
 # 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");
 #endif
   }
 
 private:
 #ifdef __x86_64__
   alignas(16) char FPState[512];
+  uint64_t Eflags;
 #endif
 };
 
 class ExegesisX86Target : public ExegesisTarget {
 public:
   ExegesisX86Target() : ExegesisTarget(X86CpuPfmCounters) {}
 
   Expected<std::unique_ptr<pfm::Counter>>