From 242f9bf3dc04170502020fb0338b78ea76b9ebac Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Thu, 30 Sep 2021 21:31:21 +0000
Subject: `std::visit` by reference

I had started the trend of doing `std::visit` by value (because a type
error once mislead me into thinking that was the only form that
existed). While the optomizer in principle should be able to deal with
extra coppying or extra indirection once the lambdas inlined, sticking
with by reference is the conventional default. I hope this might even
improve performance.
---
 src/libstore/build/local-derivation-goal.cc | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

(limited to 'src/libstore/build/local-derivation-goal.cc')

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 3aa9f12fe..e91e35851 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -1094,10 +1094,10 @@ void LocalDerivationGoal::writeStructuredAttrs()
 static StorePath pathPartOfReq(const DerivedPath & req)
 {
     return std::visit(overloaded {
-        [&](DerivedPath::Opaque bo) {
+        [&](const DerivedPath::Opaque & bo) {
             return bo.path;
         },
-        [&](DerivedPath::Built bfd)  {
+        [&](const DerivedPath::Built & bfd)  {
             return bfd.drvPath;
         },
     }, req.raw());
@@ -2155,8 +2155,8 @@ void LocalDerivationGoal::registerOutputs()
                 /* Since we'll use the already installed versions of these, we
                    can treat them as leaves and ignore any references they
                    have. */
-                [&](AlreadyRegistered _) { return StringSet {}; },
-                [&](PerhapsNeedToRegister refs) {
+                [&](const AlreadyRegistered &) { return StringSet {}; },
+                [&](const PerhapsNeedToRegister & refs) {
                     StringSet referencedOutputs;
                     /* FIXME build inverted map up front so no quadratic waste here */
                     for (auto & r : refs.refs)
@@ -2192,11 +2192,11 @@ void LocalDerivationGoal::registerOutputs()
         };
 
         std::optional<StorePathSet> referencesOpt = std::visit(overloaded {
-            [&](AlreadyRegistered skippedFinalPath) -> std::optional<StorePathSet> {
+            [&](const AlreadyRegistered & skippedFinalPath) -> std::optional<StorePathSet> {
                 finish(skippedFinalPath.path);
                 return std::nullopt;
             },
-            [&](PerhapsNeedToRegister r) -> std::optional<StorePathSet> {
+            [&](const PerhapsNeedToRegister & r) -> std::optional<StorePathSet> {
                 return r.refs;
             },
         }, outputReferencesIfUnregistered.at(outputName));
@@ -2312,7 +2312,7 @@ void LocalDerivationGoal::registerOutputs()
         };
 
         ValidPathInfo newInfo = std::visit(overloaded {
-            [&](DerivationOutputInputAddressed output) {
+            [&](const DerivationOutputInputAddressed & output) {
                 /* input-addressed case */
                 auto requiredFinalPath = output.path;
                 /* Preemptively add rewrite rule for final hash, as that is
@@ -2331,14 +2331,14 @@ void LocalDerivationGoal::registerOutputs()
                     newInfo0.references.insert(newInfo0.path);
                 return newInfo0;
             },
-            [&](DerivationOutputCAFixed dof) {
+            [&](const DerivationOutputCAFixed & dof) {
                 auto newInfo0 = newInfoFromCA(DerivationOutputCAFloating {
                     .method = dof.hash.method,
                     .hashType = dof.hash.hash.type,
                 });
 
                 /* Check wanted hash */
-                Hash & wanted = dof.hash.hash;
+                const Hash & wanted = dof.hash.hash;
                 assert(newInfo0.ca);
                 auto got = getContentAddressHash(*newInfo0.ca);
                 if (wanted != got) {
-- 
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