# Sqrt ASM code with rcc 1.2.21

**URL:** <https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109>\
**Category:** General - Software\
**Created:** [22 March 2021 08:39 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109 "2021-03-22T08:39:00Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![jeandet](https://dub1.discourse-cdn.com/flex017/user_avatar/discourse.grlib.community/jeandet/32/32_2.png) [@jeandet](https://discourse.grlib.community/u/jeandet)\
**Post date:** [22 March 2021 08:39 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/1 "2021-03-22T08:39:00Z")

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Hello,  
I’m trying to optimize some code for an instrument on Solar Orbiter. We have to do some square roots, so while reading ASM code to see some C code change effects and hunting for FPU instructions, I discovered that pattern for sqrt:

```auto
4001a9ac:	97 a0 05 2f fsqrts %f15, %f11
4001a9b0:	81 aa ca 2b fcmps %f11, %f11
4001a9b4:	01 00 00 00 nop 
4001a9b8:	13 80 00 11 fbe 4001a9fc <compute_BP1+0x144>
4001a9bc:	db 06 20 44 ld [%i0 + 0x44], %f13
4001a9c0:	df 27 bf c8 st %f15, [%fp + -56]
4001a9c4:	d1 27 bf d4 st %f8, [%fp + -44]
4001a9c8:	d3 27 bf d0 st %f9, [%fp + -48]
4001a9cc:	d5 27 bf d8 st %f10, [%fp + -40]
4001a9d0:	d9 27 bf dc st %f12, [%fp + -36]
4001a9d4:	db 27 bf e0 st %f13, [%fp + -32]
4001a9d8:	40 00 02 49 call 4001b2fc <sqrtf>

```

What I understand is that fsqrts might fail and in this case we would repeat the computation with newlib’s soft implementation. Am I right? Under which circumstances could it fail? Can we avoid it?

Best regards,  
Alexis.

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**Author:** ![mhjorth](https://avatars.discourse-cdn.com/v4/letter/m/3ec8ea/32.png) [@mhjorth](https://discourse.grlib.community/u/mhjorth)\
**Post date:** [22 March 2021 12:13 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/2 "2021-03-22T12:13:39Z")

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Hi! Are you using our binary rcc distribution or built the toolchain yourself? Any build flags (-mtune etc)? Is this generated from a single sqrtf() function call in C?

Just from looking at the disassembly, it looks like this code checks for a NaN result from fsqrts (as NaN is not equal to itself by definition) which should happen if the input to the square root is less than 0.

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<div class="post-metadata">

**Author:** ![jeandet](https://dub1.discourse-cdn.com/flex017/user_avatar/discourse.grlib.community/jeandet/32/32_2.png) [@jeandet](https://discourse.grlib.community/u/jeandet)\
**Post date:** [22 March 2021 13:02 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/3 "2021-03-22T13:02:37Z")

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Hi, I use a self built rcc since we had to modify spw init for dual link mode:  
[teamcity-docker-SolarOrbiter-LFR-agent/Dockerfile at master · jeandet/teamcity-docker-SolarOrbiter-LFR-agent · GitHub](https://github.com/jeandet/teamcity-docker-SolarOrbiter-LFR-agent/blob/master/Dockerfile)  
regarding CPU related build flags, nothing except `-mfix-b2bst`[LFR\_Flight\_Software/meson.build at R3.3 · LaboratoryOfPlasmaPhysics/LFR\_Flight\_Software · GitHub](https://github.com/LaboratoryOfPlasmaPhysics/LFR_Flight_Software/blob/R3.3/src/meson.build#L66)

NaN results was my guess, but I wondered if it could happen with allowed values(\>=0) because I would expect libc’s sqrt to return the same result (NaN) with anything \< 0.

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<div class="post-metadata">

**Author:** ![jeandet](https://dub1.discourse-cdn.com/flex017/user_avatar/discourse.grlib.community/jeandet/32/32_2.png) [@jeandet](https://discourse.grlib.community/u/jeandet)\
**Post date:** [22 March 2021 13:22 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/4 "2021-03-22T13:22:44Z")

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Maybe it is called to get ERRNO set?

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<div class="post-metadata">

**Author:** ![jeandet](https://dub1.discourse-cdn.com/flex017/user_avatar/discourse.grlib.community/jeandet/32/32_2.png) [@jeandet](https://discourse.grlib.community/u/jeandet)\
**Post date:** [25 March 2021 13:13 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/5 "2021-03-25T13:13:07Z")

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Yes it was errno 🙂

 ![Capture d’écran de 2021-03-25 14-12-09](https://europe1.discourse-cdn.com/flex017/uploads/grlib/original/1X/e2b80cda85594ba46939109223a87f0ff4c05f66.png)

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<div class="post-metadata">

**Author:** ![mhjorth](https://avatars.discourse-cdn.com/v4/letter/m/3ec8ea/32.png) [@mhjorth](https://discourse.grlib.community/u/mhjorth)\
**Post date:** [5 May 2021 08:47 UTC](https://discourse.grlib.community/t/sqrt-asm-code-with-rcc-1-2-21/109/6 "2021-05-05T08:47:52Z")

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Good find, thanks for reporting back!