From 3e1aa7cb59aff4b245b45e326fcdba1bf7f105c6 Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Fri, 6 Mar 2015 21:55:32 +0100 Subject: [PATCH] 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 Acked-by: Andy Lutomirski Acked-by: Linus Torvalds Cc: Borislav Petkov Cc: Frederic Weisbecker Cc: H. Peter Anvin Cc: H. Peter Anvin Cc: Kees Cook Cc: Oleg Nesterov Cc: Steven Rostedt Cc: Will Drewry Link: http://lkml.kernel.org/r/1425675332-31576-1-git-send-email-dvlasenk@redhat.com Signed-off-by: Ingo Molnar --- 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 9a0919678f97..ae6588b301c2 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 e13f8e7c22a6..77630d57e7bf 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 3fd2c693e475..04cb1790a596 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 c3b9953d3fa0..9bc944a91274 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 2419d5fefae3..9734182966f3 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 -- 2.39.5