From 3009566a46f19e9c94df4cf51cc374763698cba0 Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Wed, 23 Apr 2025 23:23:53 -0700
Subject: [PATCH] runtime: fix tag pointers on aix

Clean up tagged pointers a bit. I got the shifts wrong
for the weird aix case.

Change-Id: I21449fd5973f4651fd1103d3b8be9c2b9b93a490
Reviewed-on: https://go-review.googlesource.com/c/go/+/667715
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 src/runtime/malloc.go       |  4 ++--
 src/runtime/netpoll.go      |  2 +-
 src/runtime/tagptr_64bit.go | 46 +++++++++++--------------------------
 3 files changed, 16 insertions(+), 36 deletions(-)

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 010f20bf94..ffaf92debc 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -455,8 +455,8 @@ func mallocinit() {
 		throw("max pointer/scan bitmap size for headerless objects is too large")
 	}
 
-	if minTagBits > taggedPointerBits {
-		throw("taggedPointerBits too small")
+	if minTagBits > tagBits {
+		throw("tagBits too small")
 	}
 
 	// Initialize the heap.
diff --git a/src/runtime/netpoll.go b/src/runtime/netpoll.go
index b2219b92ce..fab921e2d3 100644
--- a/src/runtime/netpoll.go
+++ b/src/runtime/netpoll.go
@@ -302,7 +302,7 @@ func (c *pollCache) free(pd *pollDesc) {
 	// Increment the fdseq field, so that any currently
 	// running netpoll calls will not mark pd as ready.
 	fdseq := pd.fdseq.Load()
-	fdseq = (fdseq + 1) & (1<<taggedPointerBits - 1)
+	fdseq = (fdseq + 1) & (1<<tagBits - 1)
 	pd.fdseq.Store(fdseq)
 
 	pd.publishInfo()
diff --git a/src/runtime/tagptr_64bit.go b/src/runtime/tagptr_64bit.go
index b0e3d97223..31cbce03b7 100644
--- a/src/runtime/tagptr_64bit.go
+++ b/src/runtime/tagptr_64bit.go
@@ -17,21 +17,15 @@ const (
 	//
 	// See heapAddrBits for a table of address space sizes on
 	// various architectures. 48 bits is enough for all
-	// architectures except s390x.
+	// arch/os combos except s390x, aix, and riscv64.
 	//
-	// On AMD64, virtual addresses are 48-bit (or 57-bit) numbers sign extended to 64.
-	// We shift the address left 16 to eliminate the sign extended part and make
-	// room in the bottom for the count.
+	// On AMD64, virtual addresses are 48-bit (or 57-bit) sign-extended.
+	// Other archs are 48-bit zero-extended.
 	//
 	// On s390x, virtual addresses are 64-bit. There's not much we
 	// can do about this, so we just hope that the kernel doesn't
 	// get to really high addresses and panic if it does.
-	addrBits = 48
-
-	// In addition to the 16 bits taken from the top, we can take 9 from the
-	// bottom, because we require pointers to be well-aligned (see tagptr.go:tagAlignBits).
-	// That gives us a total of 25 bits for the tag.
-	tagBits = 64 - addrBits + tagAlignBits
+	defaultAddrBits = 48
 
 	// On AIX, 64-bit addresses are split into 36-bit segment number and 28-bit
 	// offset in segment.  Segment numbers in the range 0x0A0000000-0x0AFFFFFFF(LSA)
@@ -39,33 +33,25 @@ const (
 	// We assume all tagged addresses are from memory allocated with mmap.
 	// We use one bit to distinguish between the two ranges.
 	aixAddrBits = 57
-	aixTagBits  = 64 - aixAddrBits + tagAlignBits
 
 	// riscv64 SV57 mode gives 56 bits of userspace VA.
 	// tagged pointer code supports it,
 	// but broader support for SV57 mode is incomplete,
 	// and there may be other issues (see #54104).
 	riscv64AddrBits = 56
-	riscv64TagBits  = 64 - riscv64AddrBits + tagAlignBits
-)
 
-// The number of bits stored in the numeric tag of a taggedPointer
-const taggedPointerBits = (goos.IsAix * aixTagBits) + (goarch.IsRiscv64 * riscv64TagBits) + ((1 - goos.IsAix) * (1 - goarch.IsRiscv64) * tagBits)
+	addrBits = goos.IsAix*aixAddrBits + goarch.IsRiscv64*riscv64AddrBits + (1-goos.IsAix)*(1-goarch.IsRiscv64)*defaultAddrBits
+
+	// In addition to the 16 bits (or other, depending on arch/os) taken from the top,
+	// we can take 9 from the bottom, because we require pointers to be well-aligned
+	// (see tagptr.go:tagAlignBits). That gives us a total of 25 bits for the tag.
+	tagBits = 64 - addrBits + tagAlignBits
+)
 
 // taggedPointerPack created a taggedPointer from a pointer and a tag.
 // Tag bits that don't fit in the result are discarded.
 func taggedPointerPack(ptr unsafe.Pointer, tag uintptr) taggedPointer {
-	var t taggedPointer
-	if GOOS == "aix" {
-		if GOARCH != "ppc64" {
-			throw("check this code for aix on non-ppc64")
-		}
-		t = taggedPointer(uint64(uintptr(ptr))<<(64-aixAddrBits) | uint64(tag&(1<<aixTagBits-1)))
-	} else if GOARCH == "riscv64" {
-		t = taggedPointer(uint64(uintptr(ptr))<<(64-riscv64AddrBits) | uint64(tag&(1<<riscv64TagBits-1)))
-	} else {
-		t = taggedPointer(uint64(uintptr(ptr))<<(64-addrBits) | uint64(tag&(1<<tagBits-1)))
-	}
+	t := taggedPointer(uint64(uintptr(ptr))<<(tagBits-tagAlignBits) | uint64(tag&(1<<tagBits-1)))
 	if t.pointer() != ptr || t.tag() != tag {
 		print("runtime: taggedPointerPack invalid packing: ptr=", ptr, " tag=", hex(tag), " packed=", hex(t), " -> ptr=", t.pointer(), " tag=", hex(t.tag()), "\n")
 		throw("taggedPointerPack")
@@ -80,16 +66,10 @@ func (tp taggedPointer) pointer() unsafe.Pointer {
 		// val before unpacking.
 		return unsafe.Pointer(uintptr(int64(tp) >> tagBits << tagAlignBits))
 	}
-	if GOOS == "aix" {
-		return unsafe.Pointer(uintptr((tp >> aixTagBits << tagAlignBits) | 0xa<<56))
-	}
-	if GOARCH == "riscv64" {
-		return unsafe.Pointer(uintptr(tp >> riscv64TagBits << tagAlignBits))
-	}
 	return unsafe.Pointer(uintptr(tp >> tagBits << tagAlignBits))
 }
 
 // Tag returns the tag from a taggedPointer.
 func (tp taggedPointer) tag() uintptr {
-	return uintptr(tp & (1<<taggedPointerBits - 1))
+	return uintptr(tp & (1<<tagBits - 1))
 }