[RFC PATCH 2/4] ntdll: Consolidate 2 LDRs into 1 LDRD in ARM syscall dispatcher.

[Jinoh Kang]

LDRD is an ARMv7 instruction that loads two consecutive 32-bit words in
one operation.  A single LDRD saves one clock cycle compared to two
LDRs.

The optimization introduced by this commit may not significantly affect
performance, since the final LDM instruction may have to wait for the
previous load instruction(s) to free up the load pipeline.  However,
future modifications that reduce the load pressure from the succeding
instructions can benefit from this optimization.  Also, shortened code
leads to less time spent on instruction fetch and more efficient I-cache
utilization.

Running llvm-mca --timeline on the old code reports:
                    01234567
Index     0123456789

[0,0]     DeER .    .    . .   tst.w   r12, #2
[0,1]     D--R .    .    . .   it      ne
[0,2]     .DeeeeER  .    . .   ldmne.w r8, {r0, r1, r2, r3}
[0,3]     . D===eeeeER   . .   ldr.w   lr, [r8, #60]
[0,4]     . D====eeeeER  . .   ldr.w   sp, [r8, #56]
[0,5]     .  DeE------R  . .   add.w   r8, r8, #16
[0,6]     .   D===eeeeeeeeER   ldm.w   r8, {r4, r5, r6, r7, r8, r9, r10, r11, r12, pc}

Running llvm-mca --timeline on the new code reports:
                    01234567
Index     0123456789

[0,0]     DeER .    .    . .   tst.w   r12, #2
[0,1]     D--R .    .    . .   it      ne
[0,2]     .DeeeeER  .    . .   ldmne.w r8, {r0, r1, r2, r3}
[0,3]     . D===eeeeER   . .   ldrd    sp, lr, [r8, #56]
[0,4]     .  DeE-----R   . .   add.w   r8, r8, #16
[0,5]     .   D===eeeeeeeeER   ldm.w   r8, {r4, r5, r6, r7, r8, r9, r10, r11, r12, pc}

Signed-off-by: Jinoh Kang <jinoh.kang.kr at gmail.com>
---
 dlls/ntdll/unix/signal_arm.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/dlls/ntdll/unix/signal_arm.c b/dlls/ntdll/unix/signal_arm.c
index 8c8750153df..c63f669668c 100644
--- a/dlls/ntdll/unix/signal_arm.c
+++ b/dlls/ntdll/unix/signal_arm.c
@@ -1208,8 +1208,7 @@ __ASM_GLOBAL_FUNC( __wine_syscall_dispatcher,
                    "tst ip, #2\n\t"                 /* CONTEXT_INTEGER */
                    "it ne\n\t"
                    "ldmne r8, {r0-r3}\n\t"
-                   "ldr lr, [r8, #0x3c]\n\t"
-                   "ldr sp, [r8, #0x38]\n\t"
+                   "ldrd sp, lr, [r8, #0x38]\n\t"
                    "add r8, r8, #0x10\n\t"
                    "ldm r8, {r4-r12,pc}\n"
                    "5:\tmovw r0, #0x000d\n\t" /* STATUS_INVALID_PARAMETER */
-- 
2.35.1