[GISel] Add debug counter to force sdag fallback (#78257)

Add a debug counter that allows forcing an sdag fallback after a certain
number of functions.

The intended use-case is to bisect which function gets miscompiled by
global isel using `-debug-counter=globalisel-count=N` (in cases where
sdag doesn't also miscompile it, of course).

The "falling back" debug line is printed unconditionally, because using
`-debug-only` is usually too spammy for the intended purpose.

Author: nikic

llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp

@@ -31,12 +31,16 @@
 #include "llvm/Support/CodeGenCoverage.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/DebugCounter.h"
 #include "llvm/Target/TargetMachine.h"
 
 #define DEBUG_TYPE "instruction-select"
 
 using namespace llvm;
 
+DEBUG_COUNTER(GlobalISelCounter, "globalisel",
+              "Controls whether to select function with GlobalISel");
+
 #ifdef LLVM_GISEL_COV_PREFIX
 static cl::opt<std::string>
     CoveragePrefix("gisel-coverage-prefix", cl::init(LLVM_GISEL_COV_PREFIX),
@@ -294,6 +298,13 @@ bool InstructionSelect::runOnMachineFunction(MachineFunction &MF) {
     return false;
   }
 #endif
+
+  if (!DebugCounter::shouldExecute(GlobalISelCounter)) {
+    dbgs() << "Falling back for function " << MF.getName() << "\n";
+    MF.getProperties().set(MachineFunctionProperties::Property::FailedISel);
+    return false;
+  }
+
   // Determine if there are any calls in this machine function. Ported from
   // SelectionDAG.
   MachineFrameInfo &MFI = MF.getFrameInfo();

llvm/test/CodeGen/AArch64/GlobalISel/counter-fallback.ll

@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc -mtriple=aarch64-- -global-isel -global-isel-abort=0 -debug-counter=globalisel-count=1 %s -o - 2>/dev/null | FileCheck %s
+; RUN: llc -mtriple=aarch64-- -global-isel -global-isel-abort=0 -debug-counter=globalisel-count=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=DEBUG
+
+; REQUIRES: asserts
+
+; DEBUG-NOT: Falling back for function test1
+; DEBUG: Falling back for function test2
+
+define i32 @test1(i32 %x) {
+; CHECK-LABEL: test1:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ret
+  ret i32 %x
+}
+
+define i32 @test2(i32 %x) {
+; CHECK-LABEL: test2:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ret
+  ret i32 %x
+}
+
+;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
+; DEBUG: {{.*}}