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Create Getting Started Guide #31
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I'll writeup a wiki page describing the general structure of the backend, how it works, and what needs doing, thereby closing #5 in the process. |
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See, now if I'd seen that I wouldn't have opened this :) Closing, adding my bits there. |
dylanmckay
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that referenced
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Jul 19, 2015
- Factor out code to query and modify the sign bit of a floatingpoint
value as an integer. This also works if none of the targets integer
types is big enough to hold all bits of the floatingpoint value.
- Legalize FABS(x) as FCOPYSIGN(x, 0.0) if FCOPYSIGN is available,
otherwise perform bit manipulation on the sign bit. The previous code
used "x >u 0 ? x : -x" which is incorrect for x being -0.0! It also
takes 34 instructions on ARM Cortex-M4. With this patch we only
require 5:
vldr d0, LCPI0_0
vmov r2, r3, d0
lsrs r2, r3, #31
bfi r1, r2, #31, #1
bx lr
(This could be further improved if the compiler would recognize that
r2, r3 is zero).
- Only lower FCOPYSIGN(x, y) = sign(x) ? -FABS(x) : FABS(x) if FABS is
available otherwise perform bit manipulation on the sign bit.
- Perform the sign(x) test by masking out the sign bit and comparing
with 0 rather than shifting the sign bit to the highest position and
testing for "<s 0". For x86 copysignl (on 80bit values) this gets us:
testl $32768, %eax
rather than:
shlq $48, %rax
sets %al
testb %al, %al
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242107 91177308-0d34-0410-b5e6-96231b3b80d8
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How can I use your AVR fanciness? I'm doing some AVR work lately, so this seems like something worth playing with. I've got reasonable C experience, lots of Unix build fun experience, but about zero in the compiler development department.
Do you have some time to spend showing me how to start working on this? If I can get it working I'll MR a guide so others can pick this up faster.
Feel free to e-mail me at [email protected]
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