[llvm] [MC] Speed up checkFeatures() (NFCI) (PR #130936)

[Nikita Popov via llvm-commits]

https://github.com/nikic updated https://github.com/llvm/llvm-project/pull/130936

>From 28ca7f52bff3572a454a296283a88a80ee2ccbda Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 12 Mar 2025 10:36:41 +0100
Subject: [PATCH 1/2] [MC] Speed up checkFeatures() (NFCI)

checkFeatures() currently goes through ApplyFeatureFlag(), which
will also handle implied features. This is very slow -- just
querying every feature once takes up 10% of a Rust hello world
compile.

However, if we only want to query whether certain features are
set/unset, we can do so directly -- implied features have already
been handled when the FeatureBitset was constructed.

I've retained the existing handling for unrecognized features
(print an error and ignore them).
---
 llvm/lib/MC/MCSubtargetInfo.cpp | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/MC/MCSubtargetInfo.cpp b/llvm/lib/MC/MCSubtargetInfo.cpp
index f59ec2f7c2602..815e092deef22 100644
--- a/llvm/lib/MC/MCSubtargetInfo.cpp
+++ b/llvm/lib/MC/MCSubtargetInfo.cpp
@@ -317,14 +317,20 @@ FeatureBitset MCSubtargetInfo::ApplyFeatureFlag(StringRef FS) {
 
 bool MCSubtargetInfo::checkFeatures(StringRef FS) const {
   SubtargetFeatures T(FS);
-  FeatureBitset Set, All;
-  for (std::string F : T.getFeatures()) {
-    ::ApplyFeatureFlag(Set, F, ProcFeatures);
-    if (F[0] == '-')
-      F[0] = '+';
-    ::ApplyFeatureFlag(All, F, ProcFeatures);
-  }
-  return (FeatureBits & All) == Set;
+  return all_of(T.getFeatures(), [this](const std::string &F) {
+    assert(SubtargetFeatures::hasFlag(F) &&
+           "Feature flags should start with '+' or '-'");
+    const SubtargetFeatureKV *FeatureEntry =
+        Find(SubtargetFeatures::StripFlag(F), ProcFeatures);
+    if (!FeatureEntry) {
+      errs() << "'" << F << "' is not a recognized feature for this target"
+             << " (ignoring feature)\n";
+      return true;
+    }
+
+    return FeatureBits.test(FeatureEntry->Value) ==
+           SubtargetFeatures::isEnabled(F);
+  });
 }
 
 const MCSchedModel &MCSubtargetInfo::getSchedModelForCPU(StringRef CPU) const {

>From 22fa3c85265d2e0e3c7fddbe79b4ca6e5c35e263 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 12 Mar 2025 11:54:47 +0100
Subject: [PATCH 2/2] Use report_fatal_error instead

---
 llvm/lib/MC/MCSubtargetInfo.cpp | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/MC/MCSubtargetInfo.cpp b/llvm/lib/MC/MCSubtargetInfo.cpp
index 815e092deef22..d86eaad48420d 100644
--- a/llvm/lib/MC/MCSubtargetInfo.cpp
+++ b/llvm/lib/MC/MCSubtargetInfo.cpp
@@ -322,11 +322,9 @@ bool MCSubtargetInfo::checkFeatures(StringRef FS) const {
            "Feature flags should start with '+' or '-'");
     const SubtargetFeatureKV *FeatureEntry =
         Find(SubtargetFeatures::StripFlag(F), ProcFeatures);
-    if (!FeatureEntry) {
-      errs() << "'" << F << "' is not a recognized feature for this target"
-             << " (ignoring feature)\n";
-      return true;
-    }
+    if (!FeatureEntry)
+      report_fatal_error(Twine("'") + F +
+                         "' is not a recognized feature for this target");
 
     return FeatureBits.test(FeatureEntry->Value) ==
            SubtargetFeatures::isEnabled(F);