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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-aarch64

@llvm/pr-subscribers-llvm-globalisel

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

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 irtranslator is extremely spammy, so using LLVM_DEBUG is not viable.

---
Full diff: https://github.com/llvm/llvm-project/pull/78257.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp (+10) 
- (added) llvm/test/CodeGen/AArch64/GlobalISel/counter-fallback.ll (+25) 


``````````diff
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 662de0f3fe0e5e8..632bb9eb7360a0c 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -76,6 +76,7 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/DebugCounter.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
@@ -96,6 +97,9 @@
 
 using namespace llvm;
 
+DEBUG_COUNTER(GlobalISelCounter, "globalisel",
+              "Controls whether to select function with GlobalISel");
+
 static cl::opt<bool>
     EnableCSEInIRTranslator("enable-cse-in-irtranslator",
                             cl::desc("Should enable CSE in irtranslator"),
@@ -3701,6 +3705,12 @@ bool IRTranslator::runOnMachineFunction(MachineFunction &CurMF) {
   // Make our arguments/constants entry block fallthrough to the IR entry block.
   EntryBB->addSuccessor(&getMBB(F.front()));
 
+  if (!DebugCounter::shouldExecute(GlobalISelCounter)) {
+    dbgs() << "Falling back for function " << CurMF.getName() << "\n";
+    CurMF.getProperties().set(MachineFunctionProperties::Property::FailedISel);
+    return false;
+  }
+
   if (CLI->fallBackToDAGISel(*MF)) {
     OptimizationRemarkMissed R("gisel-irtranslator", "GISelFailure",
                                F.getSubprogram(), &F.getEntryBlock());
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/counter-fallback.ll b/llvm/test/CodeGen/AArch64/GlobalISel/counter-fallback.ll
new file mode 100644
index 000000000000000..9a39b5a2ad7a927
--- /dev/null
+++ b/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: {{.*}}

``````````

</details>


https://github.com/llvm/llvm-project/pull/78257