From 26e0660811d477dcb30f1abcf71ed9db7a9c4472 Mon Sep 17 00:00:00 2001
From: Austin Clements <austin@google.com>
Date: Mon, 7 Aug 2023 18:11:39 -0400
Subject: [PATCH] Revert "runtime: move pcvalue cache to M"

This reverts CL 515276.

This broke the longtest builders. For example:
https://build.golang.org/log/351a1a198a6b843b1881c1fb6cdef51f3e413e8b

Change-Id: Ie79067464fe8e226da31721cf127f3efb6011452
Reviewed-on: https://go-review.googlesource.com/c/go/+/516856
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
---
 src/runtime/runtime2.go |  3 ---
 src/runtime/symtab.go   | 46 ++++++++++++++++++++---------------------
 2 files changed, 22 insertions(+), 27 deletions(-)

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index e7db4dcdd7..54fab050ea 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -611,9 +611,6 @@ type m struct {
 	// Whether this is a pending preemption signal on this M.
 	signalPending atomic.Uint32
 
-	// pcvalue lookup cache
-	pcvalueCache pcvalueCache
-
 	dlogPerM
 
 	mOS
diff --git a/src/runtime/symtab.go b/src/runtime/symtab.go
index 18ba683d69..ff5f5f7f0e 100644
--- a/src/runtime/symtab.go
+++ b/src/runtime/symtab.go
@@ -843,32 +843,30 @@ func pcvalueCacheKey(targetpc uintptr) uintptr {
 }
 
 // Returns the PCData value, and the PC where this value starts.
-func pcvalue(f funcInfo, off uint32, targetpc uintptr, _ *pcvalueCache, strict bool) (int32, uintptr) {
+func pcvalue(f funcInfo, off uint32, targetpc uintptr, cache *pcvalueCache, strict bool) (int32, uintptr) {
 	if off == 0 {
 		return -1, 0
 	}
 
 	// Check the cache. This speeds up walks of deep stacks, which
-	// tend to have the same recursive functions over and over,
-	// or repetitive stacks between goroutines.
-	ck := pcvalueCacheKey(targetpc)
-	{
-		mp := acquirem()
-		cache := &mp.pcvalueCache
-		for i := range cache.entries[ck] {
+	// tend to have the same recursive functions over and over.
+	//
+	// This cache is small enough that full associativity is
+	// cheaper than doing the hashing for a less associative
+	// cache.
+	if cache != nil {
+		x := pcvalueCacheKey(targetpc)
+		for i := range cache.entries[x] {
 			// We check off first because we're more
 			// likely to have multiple entries with
 			// different offsets for the same targetpc
 			// than the other way around, so we'll usually
 			// fail in the first clause.
-			ent := &cache.entries[ck][i]
+			ent := &cache.entries[x][i]
 			if ent.off == off && ent.targetpc == targetpc {
-				val, pc := ent.val, ent.valPC
-				releasem(mp)
-				return val, pc
+				return ent.val, ent.valPC
 			}
 		}
-		releasem(mp)
 	}
 
 	if !f.valid() {
@@ -896,18 +894,18 @@ func pcvalue(f funcInfo, off uint32, targetpc uintptr, _ *pcvalueCache, strict b
 			// larger than the cache.
 			// Put the new element at the beginning,
 			// since it is the most likely to be newly used.
-			mp := acquirem()
-			cache := &mp.pcvalueCache
-			e := &cache.entries[ck]
-			ci := fastrandn(uint32(len(cache.entries[ck])))
-			e[ci] = e[0]
-			e[0] = pcvalueCacheEnt{
-				targetpc: targetpc,
-				off:      off,
-				val:      val,
-				valPC:    prevpc,
+			if cache != nil {
+				x := pcvalueCacheKey(targetpc)
+				e := &cache.entries[x]
+				ci := fastrandn(uint32(len(cache.entries[x])))
+				e[ci] = e[0]
+				e[0] = pcvalueCacheEnt{
+					targetpc: targetpc,
+					off:      off,
+					val:      val,
+					valPC:    prevpc,
+				}
 			}
-			releasem(mp)
 
 			return val, prevpc
 		}
-- 
2.50.0