[llvm] [llvm-exegesis] Preserve rcx and r11 around system call (PR #72807)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-tools-llvm-exegesis

Author: Aiden Grossman (boomanaiden154)

<details>
<summary>Changes</summary>

Currently, when making the ioctl system call, we're not preserving rcx and r11. The system call will clobber these registers, meaning that the values of the registers in the snippet will be different than expected. This patch fixes that be preserving the registers around the system call, similar to how the other registers involved in the making the system call get preserved.

Fixes #<!-- -->72741.

---
Full diff: https://github.com/llvm/llvm-project/pull/72807.diff


2 Files Affected:

- (modified) llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s (+19-13) 
- (modified) llvm/tools/llvm-exegesis/lib/X86/Target.cpp (+9-2) 


``````````diff
diff --git a/llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s b/llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s
index 320d1d7db38f422..11ae00dacda2eee 100644
--- a/llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s
+++ b/llvm/test/tools/llvm-exegesis/X86/latency/subprocess-preserved-registers.s
@@ -9,21 +9,27 @@
 # 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
+# LLVM-EXEGESIS-DEFREG RAX 3
+# LLVM-EXEGESIS-DEFREG RCX 5
+# LLVM-EXEGESIS-DEFREG RDI 7
+# LLVM-EXEGESIS-DEFREG RSI B
+# LLVM-EXEGESIS-DEFREG R11 D
+# LLVM-EXEGESIS-DEFREG R14 127
+# LLVM-EXEGESIS-DEFREG R15 0
+
+cmpq $0x3, %rax
+cmovneq %r14, %r15
+cmpq $0x5, %rcx
+cmovneq %r14, %r15
+cmpq $0x7, %rdi
+cmovneq %r14, %r15
+cmpq $0xB, %rsi
+cmovneq %r14, %r15
+cmpq $0xD, %r11
+cmovneq %r14, %r15
 
 movq $60, %rax
-movq %r13, %rdi
+movq %r15, %rdi
 syscall
 
 # CHECK-NOT: error:           'Child benchmarking process exited with non-zero exit code: Child process returned with unknown exit code'
diff --git a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
index bf8fd9ec7066478..d025fe955be5157 100644
--- a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
@@ -1185,10 +1185,14 @@ std::vector<MCInst>
 ExegesisX86Target::configurePerfCounter(long Request, bool SaveRegisters) const {
   std::vector<MCInst> ConfigurePerfCounterCode;
   if(SaveRegisters) {
-    // Preservie RAX, RDI, and RSI by pushing them to the stack.
+    // Preserve RAX, RDI, and RSI by pushing them to the stack.
     generateRegisterStackPush(X86::RAX, ConfigurePerfCounterCode);
     generateRegisterStackPush(X86::RDI, ConfigurePerfCounterCode);
     generateRegisterStackPush(X86::RSI, ConfigurePerfCounterCode);
+    // RCX and R11 will get clobbered by the syscall instruction, so save them
+    // as well.
+    generateRegisterStackPush(X86::RCX, ConfigurePerfCounterCode);
+    generateRegisterStackPush(X86::R11, ConfigurePerfCounterCode);
   }
   ConfigurePerfCounterCode.push_back(
       loadImmediate(X86::RDI, 64, APInt(64, getAuxiliaryMemoryStartAddress())));
@@ -1203,6 +1207,9 @@ ExegesisX86Target::configurePerfCounter(long Request, bool SaveRegisters) const
       loadImmediate(X86::RSI, 64, APInt(64, Request)));
   generateSyscall(SYS_ioctl, ConfigurePerfCounterCode);
   if(SaveRegisters) {
+    // Restore R11 then RCX
+    generateRegisterStackPop(X86::R11, ConfigurePerfCounterCode);
+    generateRegisterStackPop(X86::RCX, ConfigurePerfCounterCode);
     // Restore RAX, RDI, and RSI, in reverse order.
     generateRegisterStackPop(X86::RSI, ConfigurePerfCounterCode);
     generateRegisterStackPop(X86::RDI, ConfigurePerfCounterCode);
@@ -1216,7 +1223,7 @@ std::vector<unsigned> ExegesisX86Target::getArgumentRegisters() const {
 }
 
 std::vector<unsigned> ExegesisX86Target::getRegistersNeedSaving() const {
-  return {X86::RAX, X86::RDI, X86::RSI};
+  return {X86::RAX, X86::RDI, X86::RSI, X86::RCX, X86::R11};
 }
 
 #endif // __linux__

``````````

</details>


https://github.com/llvm/llvm-project/pull/72807