[tip:x86/asm] x86/asm: Optimize unnecessarily wide TEST instructions
From: tip-bot for Denys Vlasenko
Date: Sat Mar 07 2015 - 11:44:34 EST
Commit-ID: 3e1aa7cb59aff4b245b45e326fcdba1bf7f105c6
Gitweb: http://git.kernel.org/tip/3e1aa7cb59aff4b245b45e326fcdba1bf7f105c6
Author: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
AuthorDate: Fri, 6 Mar 2015 21:55:32 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 7 Mar 2015 11:12:43 +0100
x86/asm: Optimize unnecessarily wide TEST instructions
By the nature of the TEST operation, it is often possible to test
a narrower part of the operand:
"testl $3, mem" -> "testb $3, mem",
"testq $3, %rcx" -> "testb $3, %cl"
This results in shorter instructions, because the TEST instruction
has no sign-entending byte-immediate forms unlike other ALU ops.
Note that this change does not create any LCP (Length-Changing Prefix)
stalls, which happen when adding a 0x66 prefix, which happens when
16-bit immediates are used, which changes such TEST instructions:
[test_opcode] [modrm] [imm32]
to:
[0x66] [test_opcode] [modrm] [imm16]
where [imm16] has a *different length* now: 2 bytes instead of 4.
This confuses the decoder and slows down execution.
REX prefixes were carefully designed to almost never hit this case:
adding REX prefix does not change instruction length except MOVABS
and MOV [addr],RAX instruction.
This patch does not add instructions which would use a 0x66 prefix,
code changes in assembly are:
-48 f7 07 01 00 00 00 testq $0x1,(%rdi)
+f6 07 01 testb $0x1,(%rdi)
-48 f7 c1 01 00 00 00 test $0x1,%rcx
+f6 c1 01 test $0x1,%cl
-48 f7 c1 02 00 00 00 test $0x2,%rcx
+f6 c1 02 test $0x2,%cl
-41 f7 c2 01 00 00 00 test $0x1,%r10d
+41 f6 c2 01 test $0x1,%r10b
-48 f7 c1 04 00 00 00 test $0x4,%rcx
+f6 c1 04 test $0x4,%cl
-48 f7 c1 08 00 00 00 test $0x8,%rcx
+f6 c1 08 test $0x8,%cl
Linus further notes:
"There are no stalls from using 8-bit instruction forms.
Now, changing from 64-bit or 32-bit 'test' instructions to 8-bit ones
*could* cause problems if it ends up having forwarding issues, so that
instead of just forwarding the result, you end up having to wait for
it to be stable in the L1 cache (or possibly the register file). The
forwarding from the store buffer is simplest and most reliable if the
read is done at the exact same address and the exact same size as the
write that gets forwarded.
But that's true only if:
(a) the write was very recent and is still in the write queue. I'm
not sure that's the case here anyway.
(b) on at least most Intel microarchitectures, you have to test a
different byte than the lowest one (so forwarding a 64-bit write
to a 8-bit read ends up working fine, as long as the 8-bit read
is of the low 8 bits of the written data).
A very similar issue *might* show up for registers too, not just
memory writes, if you use 'testb' with a high-byte register (where
instead of forwarding the value from the original producer it needs to
go through the register file and then shifted). But it's mainly a
problem for store buffers.
But afaik, the way Denys changed the test instructions, neither of the
above issues should be true.
The real problem for store buffer forwarding tends to be "write 8
bits, read 32 bits". That can be really surprisingly expensive,
because the read ends up having to wait until the write has hit the
cacheline, and we might talk tens of cycles of latency here. But
"write 32 bits, read the low 8 bits" *should* be fast on pretty much
all x86 chips, afaik."
Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1425675332-31576-1-git-send-email-dvlasenk@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/head_64.S | 2 +-
arch/x86/kernel/relocate_kernel_32.S | 8 ++++----
arch/x86/kernel/relocate_kernel_64.S | 8 ++++----
arch/x86/lib/checksum_32.S | 4 ++--
arch/x86/lib/csum-copy_64.S | 2 +-
5 files changed, 12 insertions(+), 12 deletions(-)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9a09196..ae6588b 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -146,7 +146,7 @@ startup_64:
leaq level2_kernel_pgt(%rip), %rdi
leaq 4096(%rdi), %r8
/* See if it is a valid page table entry */
-1: testq $1, 0(%rdi)
+1: testb $1, 0(%rdi)
jz 2f
addq %rbp, 0(%rdi)
/* Go to the next page */
diff --git a/arch/x86/kernel/relocate_kernel_32.S b/arch/x86/kernel/relocate_kernel_32.S
index e13f8e7..77630d5 100644
--- a/arch/x86/kernel/relocate_kernel_32.S
+++ b/arch/x86/kernel/relocate_kernel_32.S
@@ -226,23 +226,23 @@ swap_pages:
movl (%ebx), %ecx
addl $4, %ebx
1:
- testl $0x1, %ecx /* is it a destination page */
+ testb $0x1, %cl /* is it a destination page */
jz 2f
movl %ecx, %edi
andl $0xfffff000, %edi
jmp 0b
2:
- testl $0x2, %ecx /* is it an indirection page */
+ testb $0x2, %cl /* is it an indirection page */
jz 2f
movl %ecx, %ebx
andl $0xfffff000, %ebx
jmp 0b
2:
- testl $0x4, %ecx /* is it the done indicator */
+ testb $0x4, %cl /* is it the done indicator */
jz 2f
jmp 3f
2:
- testl $0x8, %ecx /* is it the source indicator */
+ testb $0x8, %cl /* is it the source indicator */
jz 0b /* Ignore it otherwise */
movl %ecx, %esi /* For every source page do a copy */
andl $0xfffff000, %esi
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 3fd2c69..04cb179 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -221,23 +221,23 @@ swap_pages:
movq (%rbx), %rcx
addq $8, %rbx
1:
- testq $0x1, %rcx /* is it a destination page? */
+ testb $0x1, %cl /* is it a destination page? */
jz 2f
movq %rcx, %rdi
andq $0xfffffffffffff000, %rdi
jmp 0b
2:
- testq $0x2, %rcx /* is it an indirection page? */
+ testb $0x2, %cl /* is it an indirection page? */
jz 2f
movq %rcx, %rbx
andq $0xfffffffffffff000, %rbx
jmp 0b
2:
- testq $0x4, %rcx /* is it the done indicator? */
+ testb $0x4, %cl /* is it the done indicator? */
jz 2f
jmp 3f
2:
- testq $0x8, %rcx /* is it the source indicator? */
+ testb $0x8, %cl /* is it the source indicator? */
jz 0b /* Ignore it otherwise */
movq %rcx, %rsi /* For ever source page do a copy */
andq $0xfffffffffffff000, %rsi
diff --git a/arch/x86/lib/checksum_32.S b/arch/x86/lib/checksum_32.S
index c3b9953..9bc944a 100644
--- a/arch/x86/lib/checksum_32.S
+++ b/arch/x86/lib/checksum_32.S
@@ -125,7 +125,7 @@ ENTRY(csum_partial)
6: addl %ecx,%eax
adcl $0, %eax
7:
- testl $1, 12(%esp)
+ testb $1, 12(%esp)
jz 8f
roll $8, %eax
8:
@@ -245,7 +245,7 @@ ENTRY(csum_partial)
addl %ebx,%eax
adcl $0,%eax
80:
- testl $1, 12(%esp)
+ testb $1, 12(%esp)
jz 90f
roll $8, %eax
90:
diff --git a/arch/x86/lib/csum-copy_64.S b/arch/x86/lib/csum-copy_64.S
index 2419d5f..9734182 100644
--- a/arch/x86/lib/csum-copy_64.S
+++ b/arch/x86/lib/csum-copy_64.S
@@ -196,7 +196,7 @@ ENTRY(csum_partial_copy_generic)
/* handle last odd byte */
.Lhandle_1:
- testl $1, %r10d
+ testb $1, %r10b
jz .Lende
xorl %ebx, %ebx
source
--
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