[llvm-exegesis] Allow the target to disable the selection of some registers.

Summary:
This prevents "Cannot encode high byte register in REX-prefixed instruction"
from happening on instructions that require REX encoding when AH & co
get selected.
On the down side, these 4 registers can no longer be selected
automatically, but this avoids having to expose all the X86 encoding
complexity.

Reviewers: gchatelet

Subscribers: tschuett, jdoerfert, llvm-commits, bdb

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59821

llvm-svn: 357003

5 files changed, 38 insertions, 2 deletions

diff --git a/llvm/test/tools/llvm-exegesis/X86/latency-SBB8rr.s b/llvm/test/tools/llvm-exegesis/X86/latency-SBB8rr.s
new file mode 100644
index 00000000000..d98d547cbd3
--- /dev/null
+++ b/llvm/test/tools/llvm-exegesis/X86/latency-SBB8rr.s
@@ -0,0 +1,11 @@
+# RUN: llvm-exegesis -mode=latency -opcode-name=SBB8rr | FileCheck %s
+
+CHECK:      ---
+CHECK-NEXT: mode: latency
+CHECK-NEXT: key:
+CHECK-NEXT:   instructions:
+CHECK-NEXT:     SBB8rr
+CHECK-NEXT: config: ''
+CHECK-NEXT: register_initial_values:
+CHECK-DAG: - '[[REG1:[A-Z0-9]+]]=0x0'
+CHECK-LAST: ...
diff --git a/llvm/tools/llvm-exegesis/lib/LlvmState.cpp b/llvm/tools/llvm-exegesis/lib/LlvmState.cpp
index ab487fd3897..059e8cb8c60 100644
--- a/llvm/tools/llvm-exegesis/lib/LlvmState.cpp
+++ b/llvm/tools/llvm-exegesis/lib/LlvmState.cpp
@@ -38,8 +38,11 @@ LLVMState::LLVMState(const std::string &Triple, const std::string &CpuName,
   }
   PfmCounters = &TheExegesisTarget->getPfmCounters(CpuName);
 
-  RATC.reset(new RegisterAliasingTrackerCache(
-      getRegInfo(), getFunctionReservedRegs(getTargetMachine())));
+  BitVector ReservedRegs = getFunctionReservedRegs(getTargetMachine());
+  for (const unsigned Reg : TheExegesisTarget->getUnavailableRegisters())
+    ReservedRegs.set(Reg);
+  RATC.reset(
+      new RegisterAliasingTrackerCache(getRegInfo(), std::move(ReservedRegs)));
   IC.reset(new InstructionsCache(getInstrInfo(), getRATC()));
 }
 
diff --git a/llvm/tools/llvm-exegesis/lib/Target.h b/llvm/tools/llvm-exegesis/lib/Target.h
index f3429b79a34..ab760bfa820 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.h
+++ b/llvm/tools/llvm-exegesis/lib/Target.h
@@ -90,6 +90,11 @@ public:
         "fillMemoryOperands() requires getScratchMemoryRegister() > 0");
   }
 
+  // Returns a list of unavailable registers.
+  // Targets can use this to prevent some registers to be automatically selected
+  // for use in snippets.
+  virtual ArrayRef<unsigned> getUnavailableRegisters() const { return {}; }
+
   // Returns the maximum number of bytes a load/store instruction can access at
   // once. This is typically the size of the largest register available on the
   // processor. Note that this only used as a hint to generate independant
diff --git a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
index 2d705c9d18a..369ed2f97d7 100644
--- a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
@@ -435,6 +435,12 @@ private:
                                      unsigned Reg,
                                      const llvm::APInt &Value) const override;
 
+  ArrayRef<unsigned> getUnavailableRegisters() const override {
+    return makeArrayRef(kUnavailableRegisters,
+                        sizeof(kUnavailableRegisters) /
+                            sizeof(kUnavailableRegisters[0]));
+  }
+
   std::unique_ptr<SnippetGenerator>
   createLatencySnippetGenerator(const LLVMState &State) const override {
     return llvm::make_unique<X86LatencySnippetGenerator>(State);
@@ -448,7 +454,14 @@ private:
   bool matchesArch(llvm::Triple::ArchType Arch) const override {
     return Arch == llvm::Triple::x86_64 || Arch == llvm::Triple::x86;
   }
+
+  static const unsigned kUnavailableRegisters[4];
 };
+
+// We disable a few registers that cannot be encoded on instructions with a REX
+// prefix.
+const unsigned ExegesisX86Target::kUnavailableRegisters[4] = {X86::AH, X86::BH,
+                                                              X86::CH, X86::DH};
 } // namespace
 
 void ExegesisX86Target::addTargetSpecificPasses(
diff --git a/llvm/unittests/tools/llvm-exegesis/X86/TargetTest.cpp b/llvm/unittests/tools/llvm-exegesis/X86/TargetTest.cpp
index 15a1d8381fd..344ec6027d2 100644
--- a/llvm/unittests/tools/llvm-exegesis/X86/TargetTest.cpp
+++ b/llvm/unittests/tools/llvm-exegesis/X86/TargetTest.cpp
@@ -144,6 +144,10 @@ public:
   Core2Avx512TargetTest() : X86TargetTest("+avx512vl") {}
 };
 
+TEST_F(Core2TargetTest, NoHighByteRegs) {
+  EXPECT_TRUE(State.getRATC().reservedRegisters().test(X86::AH));
+}
+
 TEST_F(Core2TargetTest, SetFlags) {
   const unsigned Reg = llvm::X86::EFLAGS;
   EXPECT_THAT(