Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Real time embedded FreeRTOS mailing list 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem TCP & FAT Training


GCC/ARM_CM4F wasted word on main stack

Posted by hawk777 on November 4, 2013

Hi! Looking at the GCC/ARM_CM4F target port.c xPortPendSVHandler, I believe a word of stack space is wasted pushing r14 (the link register) on the main stack, this being done on line 449 in the 7.5.3 release. It looks like this code was copied from the CM3 target, where the push of r14 in that instruction made sense, as the exception return value was the same for all tasks and so could simply be saved on the main stack between call and return of vTaskSwitchContext. In the CM4F, on the other hand, different tasks have different exception return values depending on whether or not the task uses the FPU; as such, the push on line 445 saving the callee-save integer registers onto the process stack was extended to include the link register, r14, and the corresponding load on line 460 updated to match. However, because r14 is saved on the process stack, I believe there is no longer any reason to save it on the main stack on line 449 and restore it on line 455; certainly the LR is never used between line 455 when restored the first time and line 460 when restored the second time! This is just wasting a word of main stack space, and a little bit of CPU time, for on benefit, as far as I can tell.

GCC/ARM_CM4F wasted word on main stack

Posted by richardbarry on November 4, 2013

Looks like you could be right. I will test it with the additional push/pop removed.


[ Back to the top ]    [ About FreeRTOS ]    [ Sitemap ]    [ ]

Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2016 Real Time Engineers Ltd.
Any and all data, files, source code, html content and documentation included in the FreeRTOSTM distribution or available on this site are the exclusive property of Real Time Engineers Ltd.. See the files license.txt (included in the distribution) and this copyright notice for more information. FreeRTOSTM and FreeRTOS.orgTM are trade marks of Real Time Engineers Ltd.

Latest News:

FreeRTOS V9.0.0 is now available for download.

Free TCP/IP and file system demos for the RTOS

Sponsored Links

⇓ Now With No Code Size Limit! ⇓
⇑ Free Download Without Registering ⇑

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Renesas Electronics Gold Alliance RTOS Partner.jpg

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

Atmel RTOS partner supporting ARM Cortex-M3 and AVR32 microcontrollers

STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0

Xilinx Microblaze and Zynq partner

Silicon Labs low power RTOS partner

Altera RTOS partner for Nios II and Cortex-A9 SoC

Freescale Alliance RTOS Member supporting ARM and ColdFire microcontrollers

Infineon ARM Cortex-M microcontrollers

Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers

Cypress RTOS partner supporting ARM Cortex-M3

Fujitsu RTOS partner supporting ARM Cortex-M3 and FM3

Microsemi (previously Actel) RTOS partner supporting ARM Cortex-M3

Atollic Partner

IAR Partner

Keil ARM Partner

Embedded Artists