Messages in this thread | | | Date | Mon, 15 Jul 2013 11:40:04 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [LLVMdev] [PATCH] x86/asm: avoid mnemonics without type suffix |
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On 07/14/2013 12:49 PM, Linus Torvalds wrote: > On Sun, Jul 14, 2013 at 12:30 PM, Tim Northover <t.p.northover@gmail.com> wrote: >> >> I don't think you've actually tested that, have you? (x86-64) > > Oh, you're right, for constants > 5 bits you have that other thing > going on. I didn't think about the fact that the constant changed in > the middle of the thread (it started out as 1). > > We use the gcc constraint "I" (0-31) in the kernel for this reason. > > Linus
This is also why the Intel manuals point out that "some assemblers" can take things like:
bt[l] $63,(%rsi)
... and turn it into:
btl $31,4(%rsi)
This is definitely the friendly thing to do toward the human programmer. Unfortunately gas doesn't, nor does e.g. NASM.
-hpa
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