[llvm] Revert "[llvm-exegesis] Implement the loop repetition mode for AArch64" (PR #155423)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer created https://github.com/llvm/llvm-project/pull/155423

I see some build bot failures:
- https://lab.llvm.org/buildbot/#/builders/76/builds/12434/steps/4/logs/stdio
- https://lab.llvm.org/buildbot/#/builders/55/builds/16251

Reverts llvm/llvm-project#154751 while I investigate this.

>From 674e7a35ec3682f1475db91dfe2ed00a6e580f59 Mon Sep 17 00:00:00 2001
From: Sjoerd Meijer <smeijer at nvidia.com>
Date: Tue, 26 Aug 2025 15:24:10 +0100
Subject: [PATCH] Revert "[llvm-exegesis] Implement the loop repetition mode
 for AArch64 (#154751)"

This reverts commit 8e4d2b52a7c929c3d93745fc6c81499985aca903.
---
 .../llvm-exegesis/AArch64/loop-register.s     | 17 -----------
 .../llvm-exegesis/lib/AArch64/Target.cpp      | 30 -------------------
 2 files changed, 47 deletions(-)
 delete mode 100644 llvm/test/tools/llvm-exegesis/AArch64/loop-register.s

diff --git a/llvm/test/tools/llvm-exegesis/AArch64/loop-register.s b/llvm/test/tools/llvm-exegesis/AArch64/loop-register.s
deleted file mode 100644
index e1ffe462b50c1..0000000000000
--- a/llvm/test/tools/llvm-exegesis/AArch64/loop-register.s
+++ /dev/null
@@ -1,17 +0,0 @@
-REQUIRES: aarch64-registered-target, asserts
-
-RUN: llvm-exegesis -mcpu=neoverse-v2 --use-dummy-perf-counters --mode=latency --debug-only=print-gen-assembly --opcode-name=ADDVv4i16v -repetition-mode=loop 2>&1 | FileCheck %s
-
-CHECK:        0:  {{.*}}  str     x19, [sp, #-16]!
-CHECK-NEXT:   4:  {{.*}}  movi    d[[REG:[0-9]+]], #0000000000000000
-CHECK-NEXT:   8:  {{.*}}  mov     x19, #10000
-CHECK-NEXT:   c:  {{.*}}  nop
-CHECK-NEXT:   10: {{.*}}  nop
-CHECK-NEXT:   14: {{.*}}  nop
-CHECK-NEXT:   18: {{.*}}  nop
-CHECK-NEXT:   1c: {{.*}}  nop
-CHECK-NEXT:   20: {{.*}}  addv    h[[REG]], v[[REG]].4h
-CHECK-NEXT:   24: {{.*}}  subs    x19, x19, #1
-CHECK-NEXT:   28: {{.*}}  b.ne    #-8
-CHECK-NEXT:   2c: {{.*}}  ldr     x19, [sp], #16
-CHECK-NEXT:   30: {{.*}}  ret
diff --git a/llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp b/llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp
index c4ad9ae201d4e..3a0021e3c132d 100644
--- a/llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp
@@ -8,7 +8,6 @@
 #include "../Target.h"
 #include "AArch64.h"
 #include "AArch64RegisterInfo.h"
-#include "llvm/CodeGen/MachineInstrBuilder.h"
 
 #if defined(__aarch64__) && defined(__linux__)
 #include <sys/prctl.h> // For PR_PAC_* constants
@@ -110,10 +109,6 @@ static MCInst loadFPImmediate(MCRegister Reg, unsigned RegBitWidth,
 
 namespace {
 
-// Use X19 as the loop counter register since it's a callee-saved register
-// that's available for temporary use.
-constexpr const MCPhysReg kDefaultLoopCounterReg = AArch64::X19;
-
 class ExegesisAArch64Target : public ExegesisTarget {
 public:
   ExegesisAArch64Target()
@@ -146,31 +141,6 @@ class ExegesisAArch64Target : public ExegesisTarget {
     errs() << "setRegTo is not implemented, results will be unreliable\n";
     return {};
   }
-  MCRegister getDefaultLoopCounterRegister(const Triple &) const override {
-    return kDefaultLoopCounterReg;
-  }
-
-  void decrementLoopCounterAndJump(MachineBasicBlock &MBB,
-                                   MachineBasicBlock &TargetMBB,
-                                   const MCInstrInfo &MII,
-                                   MCRegister LoopRegister) const override {
-    // subs LoopRegister, LoopRegister, #1
-    BuildMI(&MBB, DebugLoc(), MII.get(AArch64::SUBSXri))
-        .addDef(LoopRegister)
-        .addUse(LoopRegister)
-        .addImm(1)  // Subtract 1
-        .addImm(0); // No shift amount
-    // b.ne TargetMBB
-    BuildMI(&MBB, DebugLoc(), MII.get(AArch64::Bcc))
-        .addImm(AArch64CC::NE)
-        .addMBB(&TargetMBB);
-  }
-
-  // Registers that should not be selected for use in snippets.
-  const MCPhysReg UnavailableRegisters[1] = {kDefaultLoopCounterReg};
-  ArrayRef<MCPhysReg> getUnavailableRegisters() const override {
-    return UnavailableRegisters;
-  }
 
   bool matchesArch(Triple::ArchType Arch) const override {
     return Arch == Triple::aarch64 || Arch == Triple::aarch64_be;