[llvm-exegesis] Fix stack pointer register restoration

9eb80ab changed the method for stack
pointer restoration to fix segmentation faults. However, I made a
mistake in the patch and swapped a != for a ==, which caused an
arbitrary register (the first one specified) to get restored rather than
the stack pointer specifically. This patch fixes that issue and adds
test coverage to prevent regression.

Author: boomanaiden154

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

@@ -7,21 +7,48 @@
 
 # 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 RDX 7
+# LLVM-EXEGESIS-DEFREG RBX B
+# LLVM-EXEGESIS-DEFREG RSI D
+# LLVM-EXEGESIS-DEFREG RDI 11
+# LLVM-EXEGESIS-DEFREG RSP 13
+# LLVM-EXEGESIS-DEFREG RBP 17
+# LLVM-EXEGESIS-DEFREG R8 1D
+# LLVM-EXEGESIS-DEFREG R9 1F
+# LLVM-EXEGESIS-DEFREG R10 29
+# LLVM-EXEGESIS-DEFREG R11 2B
+# LLVM-EXEGESIS-DEFREG R12 2F
+# LLVM-EXEGESIS-DEFREG R13 35
 # 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
+cmpq $0x7, %rdx
 cmovneq %r14, %r15
-cmpq $0xB, %rsi
+cmpq $0xB, %rbx
 cmovneq %r14, %r15
-cmpq $0xD, %r11
+cmpq $0xD, %rsi
+cmovneq %r14, %r15
+cmpq $0x11, %rdi
+cmovneq %r14, %r15
+cmpq $0x13, %rsp
+cmovneq %r14, %r15
+cmpq $0x17, %rbp
+cmovneq %r14, %r15
+cmpq $0x1d, %r8
+cmovneq %r14, %r15
+cmpq $0x1f, %r9
+cmovneq %r14, %r15
+cmpq $0x29, %r10
+cmovneq %r14, %r15
+cmpq $0x2b, %r11
+cmovneq %r14, %r15
+cmpq $0x2f, %r12
+cmovneq %r14, %r15
+cmpq $0x35, %r13
 cmovneq %r14, %r15
 
 movq $60, %rax

llvm/tools/llvm-exegesis/lib/Assembler.cpp

@@ -97,7 +97,7 @@ static bool generateSnippetSetupCode(
       // Load in the stack register now as we're done using it elsewhere
       // and need to set the value in preparation for executing the
       // snippet.
-      if (RV.Register == StackPointerRegister)
+      if (RV.Register != StackPointerRegister)
         continue;
       const auto SetRegisterCode = ET.setRegTo(*MSI, RV.Register, RV.Value);
       if (SetRegisterCode.empty())