[llvm-exegesis] Fix preservation of RDI in subprocess mode (#72458)

I made a typo at some point while doing the subprocess implementation,
trying to pop RIP from the stack. For whatever reason, this ends up as
popq %rax after being JITed, which means we aren't properly preserving
the value of %rdi.

Regression test added.

This fixes #72188.

Author: boomanaiden154

llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s

@@ -0,0 +1,30 @@
+# REQUIRES: exegesis-can-execute-x86_64
+
+# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -snippets-file=%s -execution-mode=subprocess | FileCheck %s
+
+# See comment in ./subprocess-abnormal-exit-code.s on the transient
+# PTRACE_ATTACH failure.
+# ALLOW_RETRIES: 2
+
+# Check that the value of the registers preserved in subprocess mode while
+# making the ioctl system call are actually preserved correctly.
+
+# LLVM-EXEGESIS-DEFREG RAX 11
+# LLVM-EXEGESIS-DEFREG RDI 13
+# LLVM-EXEGESIS-DEFREG RSI 17
+# LLVM-EXEGESIS-DEFREG R13 0
+# LLVM-EXEGESIS-DEFREG R12 127
+
+cmpq $0x11, %rax
+cmovneq %r12, %r13
+cmpq $0x13, %rdi
+cmovneq %r12, %r13
+cmpq $0x17, %rsi
+cmovneq %r12, %r13
+
+movq $60, %rax
+movq %r13, %rdi
+syscall
+
+# CHECK-NOT: error:           'Child benchmarking process exited with non-zero exit code: Child process returned with unknown exit code'
+

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

@@ -1205,7 +1205,7 @@ ExegesisX86Target::configurePerfCounter(long Request, bool SaveRegisters) const
   if(SaveRegisters) {
     // Restore RAX, RDI, and RSI, in reverse order.
     generateRegisterStackPop(X86::RSI, ConfigurePerfCounterCode);
-    generateRegisterStackPop(X86::RIP, ConfigurePerfCounterCode);
+    generateRegisterStackPop(X86::RDI, ConfigurePerfCounterCode);
     generateRegisterStackPop(X86::RAX, ConfigurePerfCounterCode);
   }
   return ConfigurePerfCounterCode;