Meaning of @ zero_extendqisi2

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I was wondering what the actual meaning of @ zero_extendqisi2 in gcc assembly output was and also the usage. I couldn't find what qisi stands for or anything along those lines.

For context, the line is ldrb r3, [fp, #-9] @ zero_extendqisi2 and this is ARM on a Raspberry Pi Zero W, compiled with GCC. For example, when reloading an unsigned char with conversion to int, with optimization disabled, with GCC9.2 with no options. https://godbolt.org/z/7xnfqh. Older GCC all the way to the earliest on Godbolt (4.5) and presumably earlier print the same comment.

arm assembly gcc c
1 Answers

This is an RTL instruction name, included in the Standard Names list of the GCC internals manual under zero_extendmn2. Here m,n are the machine modes qi and si, which are respectively a byte and a 32-bit integer. So this is GCC's indication that it is generating an instruction which takes a byte (here loaded from memory) and zero-extends it into a 32-bit integer (here in the register r3). Which is exactly what the ARM ldrb instruction does.

I don't know what the 2 stands for, but it's apparently part of GCC's naming convention.

As Peter points out, it's a little odd that GCC would include such a comment in the assembly without -fverbose-asm. Indeed the comment is coded in as part of the template string in the machine description file, arm.md. It could have been a debugging aid that some GCC developer added and then forgot to take out.

(If you submit this for your assignment, please cite this post properly.)

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