Reduce total code size by 53 bytes (big deal, right?)HEADmaster

1 files changed, 15 insertions, 11 deletions

diff --git a/src/lock_acquire.asm b/src/lock_acquire.asm
index f32ba6a..8415d7f 100644
--- a/src/lock_acquire.asm
+++ b/src/lock_acquire.asm
@@ -43,12 +43,15 @@ section .text
 ; Returns zero on success, or a standard error code.
 global linen_lock_acquire
 linen_lock_acquire:
+		; It's handy to have a register that's 0 during most of this function
+		xor esi, esi
+
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 		;;;; Check validity of argument ;;;;
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-		; Return EINVAL if rdi is NULL or otherwise invalid
-		mov eax, -22 ; (EINVAL = -22)
+		; Return EINVAL (-22) if rdi is NULL or otherwise invalid
+		lea eax, [rsi - 22] ; mov eax, -22
 
 		test rdi, rdi
 		jz acquire_return ; rdi is NULL
@@ -57,8 +60,7 @@ linen_lock_acquire:
 		; if that assumption is wrong we'll get a segmentation fault.
 		; But we don't yet trust that [rdi] is a valid lock handle!
 		; To verify this we check the canary value stored at [rdi + 8].
-		mov ecx, [rdi + 8]
-		cmp ecx, 0xCAFEBABE
+		cmp dword [rdi + 8], 0xCAFEBABE ; Oh CISC...
 		jnz acquire_return
 
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -70,7 +72,8 @@ linen_lock_acquire:
 		; See: man 2 gettid
 
 		; gettid: rax = system call ID
-		mov eax, SYS_GETTID
+		xor eax, eax
+		mov al, SYS_GETTID
 		; gettid: rax = gettid()
 		syscall
 
@@ -119,7 +122,7 @@ linen_lock_acquire:
 		; so in most real-world cases you can delete this with no downside.
 
 		; Loop counter
-		mov ecx, 10
+		mov sil, 10
 	acquire_spinloop:
 		; The "pause" instruction is specially designed for loops like this
 		; and conserves power. It causes a small delay (makes sense here).
@@ -133,7 +136,7 @@ linen_lock_acquire:
 		jz acquire_success
 
 		; Decrement loop counter until zero
-		dec ecx
+		dec esi
 		jnz acquire_spinloop
 
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -150,24 +153,25 @@ linen_lock_acquire:
 		; futex: rsi = futex_op: which futex operation we want:
 		; - FUTEX_LOCK_PI:      block until lock's owner uses FUTEX_UNLOCK_PI
 		; - FUTEX_PRIVATE_FLAG: this lock isn't shared with another process
-		mov esi, (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG)
+		mov sil, (FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG)
 		; futex: r10 = timeout: in case we had a deadline (we don't)
 		xor r10, r10
 		; futex: rdx = val:   ignored when FUTEX_LOCK_PI is used
 		; futex: r8 = uaddr2: ignored when FUTEX_LOCK_PI is used
 		; futex: r9 = val3:   ignored when FUTEX_LOCK_PI is used
 		; futex: rax = system call ID
-		mov eax, SYS_FUTEX
+		xor eax, eax
+		mov al, SYS_FUTEX
 		; futex: rax = futex(rdi, rsi, (rdx), r10, (r8), (r9))
 		syscall
  
 		; Sometimes the lock is released after the "lock cmpxchg" instruction
 		; but just before the futex call. In that case, futex returns EAGAIN.
-		cmp rax, -11 ; (-EAGAIN)
+		cmp eax, -11 ; (-EAGAIN)
 		je acquire_futex
 
 		; Any other negative return value means failure
-		test rax, rax
+		test eax, eax
 		jnz acquire_return
 
 		; Indicate that we made a futex call (see below for why)