Messages in this thread | | | Date | Fri, 9 Dec 2016 17:25:00 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86: Optimize TIF checking in __switch_to_xtra. |
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On Fri, 9 Dec 2016, Thomas Gleixner wrote: > On Tue, 6 Dec 2016, Kyle Huey wrote: > > I propose the following patch, which results in GCC generating this code: > > > > mov (%rsi),%rax > > xor (%rdi),%rax > > mov %rdi,%rbx > > mov %rsi,%r12 > > test $0x2000000,%eax > > mov %rax,%rdx > > jne 3f > > 1: test $0x10000,%edx > > je 2f > > /* do some more work */ > > 2: /* the rest of the function */ > > ret > > 3: /* do work */ > > mov (%r12),%rdx > > xor (%rbx),%rdx > > jmp 1b > > > > When we are not doing the work controlled by these flags this is significantly > > better. We only have to load the flags from memory once, and we do a single > > XOR that we can test multiple times instead of repeatedly isolating bits. > > You rely on the compiler doing the right thing. And it's non obvious from > the code that this gets optimized as above.
On i386 this creates worse code due to register pressure it seems.
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