GCC

Contents

• Current toolchain
• Analyzing compiler output
  • Compiler Explorer
  • List and symbol files
  • Fix broken objdump for Teensy 4
• Using different toolchains
  • Arduino IDE
  • VisualTeensy
  • PlatformIO

Current toolchain

Teensyduino uses the gcc-arm-none-eabi toolchain to compile and build firmware for the ARM Teensies. Current binaries of the toolchain are hosted by ARM and be downloaded here. Versions below 6.0 can be downloaded from Launchpad.

Currently Teensyduino uses the version gcc-arm-none-eabi-5_4 for all ARM based Teensy boards (T3.x and T4.x).

Analyzing compiler output

Compiler explorer

Godbolt is a useful tools which shows you the Assembler-code of short code snippets. You can compare different compiler versions, too.

• https://godbolt.org/z/i4dL_i (GCC 5.4, AVR - Teensy 2.x)
• https://godbolt.org/z/y4oh33 (GCC 5.4.1, Arm-Cortex M0+ - Teensy LC)
• https://godbolt.org/z/uVo6fG (GCC 5.4.1, Arm-Cortex M4 - Teensy 3.x)
• https://godbolt.org/z/Qc5dNE (GCC 5.4.1, Arm-Cortex M7 - Teensy 4)

List and symbol files:

Teensyduino produces additional files, which are useful if you want to dig more in to the internals or for optimizing.

In principle this should be possible with any of the build systems.Teensyduino creates an .LST and a .SYM file in the build directory. There is nothing that can't be improved - the generated .SYM file is a bit confusing because the tool nm is not used. But we can easily change that:

Example for Teensy 4:

1. Add a line to boards.txt:
   teensy40.build.command.nm=arm-none-eabi-gcc-nm

2. Add a line to platform.txt:

recipe.hooks.postbuild.4.pattern="{compiler.path}stdout_redirect" "{build.path}/{build.project_name}.symnm" "compiler.path}{build.toolchain}{build.command.nm}" -n -S --defined-only -C "{build.path}/{build.project_name}.elf"

Now, when we do a new build, an additional .symnm file is created, which is clearer and sorted -- and therefore easier to read.

Fix broken objdump for Teensy 4

The objdump for the current GCC is broken and emits more or less unreadable lst files. Copying an objdump from a newer (GCC 9) toolchain to the GCC folder fixes this. Here the output of a working objdump

00000060 <setup>:
#include "Arduino.h"

void setup()
{
  pinMode(LED_BUILTIN,OUTPUT);
      60:	movs	r1, #1
      62:	movs	r0, #13
      64:	b.w	150 <pinMode>

00000068 <loop>:
			return (CORE_PIN13_PINREG & CORE_PIN13_BITMASK) ? 1 : 0;
      68:	ldr	r3, [pc, #36]	; (90 <loop+0x28>)
      6a:	ldr	r2, [r3, #8]
      6c:	tst.w	r2, #8
				CORE_PIN13_PORTCLEAR = CORE_PIN13_BITMASK;
      70:	mov.w	r2, #8
			return (CORE_PIN13_PINREG & CORE_PIN13_BITMASK) ? 1 : 0;
      74:	bne.n	82 <loop+0x1a>
}

void loop()
{
  digitalWriteFast(LED_BUILTIN,!digitalReadFast(LED_BUILTIN));
  delay(500);
      76:	mov.w	r0, #500	; 0x1f4
				CORE_PIN13_PORTSET = CORE_PIN13_BITMASK;
      7a:	str.w	r2, [r3, #132]	; 0x84
      7e:	b.w	94 <delay>
      82:	mov.w	r0, #500	; 0x1f4
				CORE_PIN13_PORTCLEAR = CORE_PIN13_BITMASK;
      86:	str.w	r2, [r3, #136]	; 0x88
      8a:	b.w	94 <delay>
      8e:	nop
      90:	.word	0x42004000

See https://forum.pjrc.com/threads/58814-Code-generated-for-Teensy40?p=224844&viewfull=1#post224844

Using different Toolchains

Newer Toolchains are not always better. The one Teensyduino uses (5.4) is quite old - but good. If you want to try a newer one, that's easily doable:

Arduino IDE

1. There is a directory "arm" under Arduino/hardware/tools - copy the whole directory and name it i.e. "arm9" - in result, you should have two directories now, "arm" and "arm9"
2. Download (the zip-file) a newer toolchain from https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads
3. Extract it to the new directory - choose overwrite to overwrite the old version (additional files from the original version will stay)
4. Edit bords.txt. There is a line teensy40.build.toolchain=arm/bin/ - change that to teensy40.build.toolchain=arm9/bin/
5. done :)

VisualTeensy

TBD

PlatformIO

?