From c75591bfed2025a61d46240e3c57aa8fbf4ccf33 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C5=8Dshin?= <git@wholezero.org>
Date: Tue, 19 Dec 2023 13:20:11 -0500
Subject: [PATCH] Save a redundant load in zipos read/seek

When h->pos has changed to something other than SIZE_MAX, we don't need
the extra atomic load.
---
 libc/runtime/zipos-read.c | 16 +++++++---------
 libc/runtime/zipos-seek.c | 16 +++++++---------
 2 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/libc/runtime/zipos-read.c b/libc/runtime/zipos-read.c
index edc6ceab1..a7ca7c8be 100644
--- a/libc/runtime/zipos-read.c
+++ b/libc/runtime/zipos-read.c
@@ -38,17 +38,15 @@ static ssize_t __zipos_read_impl(struct ZiposHandle *h, const struct iovec *iov,
     return eisdir();
   }
   if (opt_offset == -1) {
-    while (true) {
-      start_pos = atomic_load_explicit(&h->pos, memory_order_relaxed);
+  Restart:
+    start_pos = atomic_load_explicit(&h->pos, memory_order_relaxed);
+    do {
       if (UNLIKELY(start_pos == SIZE_MAX)) {
-        continue;
+        goto Restart;
       }
-      if (LIKELY(atomic_compare_exchange_weak_explicit(
-              &h->pos, &start_pos, SIZE_MAX, memory_order_acquire,
-              memory_order_relaxed))) {
-        break;
-      }
-    }
+    } while (!LIKELY(atomic_compare_exchange_weak_explicit(
+        &h->pos, &start_pos, SIZE_MAX, memory_order_acquire,
+        memory_order_relaxed)));
     x = y = start_pos;
   } else {
     x = y = opt_offset;
diff --git a/libc/runtime/zipos-seek.c b/libc/runtime/zipos-seek.c
index e76566a02..d12efc694 100644
--- a/libc/runtime/zipos-seek.c
+++ b/libc/runtime/zipos-seek.c
@@ -47,10 +47,11 @@ static int64_t Seek(int64_t pos, int64_t offset) {
  */
 int64_t __zipos_seek(struct ZiposHandle *h, int64_t offset, unsigned whence) {
   int64_t pos, new_pos;
-  while (true) {
-    pos = atomic_load_explicit(&h->pos, memory_order_relaxed);
+Restart:
+  pos = atomic_load_explicit(&h->pos, memory_order_relaxed);
+  do {
     if (UNLIKELY(pos == SIZE_MAX)) {
-      continue;
+      goto Restart;
     }
     switch (whence) {
       case SEEK_SET:
@@ -65,11 +66,8 @@ int64_t __zipos_seek(struct ZiposHandle *h, int64_t offset, unsigned whence) {
       default:
         new_pos = einval();
     }
-    if (LIKELY(atomic_compare_exchange_weak_explicit(
-            &h->pos, &pos, new_pos < 0 ? pos : new_pos, memory_order_release,
-            memory_order_relaxed))) {
-      break;
-    }
-  }
+  } while (!LIKELY(atomic_compare_exchange_weak_explicit(
+      &h->pos, &pos, new_pos < 0 ? pos : new_pos, memory_order_release,
+      memory_order_relaxed)));
   return new_pos;
 }