From: Dmitry Selyutin Date: Mon, 9 Aug 2021 18:30:59 +0000 (+0300) Subject: docs/firststeps: markdown/pseudocode chapters X-Git-Tag: DRAFT_SVP64_0_1~458 X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=b518da5c98111e8f7480ac9bff67df8f74596d0d;p=libreriscv.git docs/firststeps: markdown/pseudocode chapters --- diff --git a/docs/firststeps.mdwn b/docs/firststeps.mdwn index db6fd78dd..eead51962 100644 --- a/docs/firststeps.mdwn +++ b/docs/firststeps.mdwn @@ -189,13 +189,58 @@ instruction, but the crucial part is quite simple: The part of the most interest is `info.func` call; we'll take a look at it in the next chapter. -# TODO info.func +# Markdown enters the scene -about info.func, info.form etc. +If we investigate the `info.func` object, we'll discover that, in case of our +`add` instruction, it is actually `fixedarith.op_add` function. Where does it +come from? -# TODO Modifying the pseudocode +This function is defined inside `fixedarith.py`, which is generated from the +corresponding Markdown file, `fixedarith.mdwn`. On the first glance, one can +find an idea to generate the code from the documentation somewhat surprising; +however, this is perfectly reasonable, since we try to stay as close to the +documentation as possible. Some Markdown files, like `fixedarith.mdwn`, +contain the pseudocode in exactly the same form as it is present in the ISA +docs. Let's take a look at `fixedarith.mdwn` and find our instruction. -TODO. search for the [Add section](https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=openpower/isa/fixedarith.mdwn;hb=HEAD) with pseudocode: +As we can see in `# Add` section, there are four `add` variants: + +* add RT,RA,RB (OE=0 Rc=0) +* add. RT,RA,RB (OE=0 Rc=1) +* addo RT,RA,RB (OE=1 Rc=0) +* addo. RT,RA,RB (OE=1 Rc=1) + +All variants are covered in the relevant OpenPOWER ISA. As of time of writing, +the most recent edition is 3.0C, and it is available here: + +https://ftp.libre-soc.org/PowerISA_public.v3.0C.pdf + +The instructions of our interest can be found exactly where we'd expect them +to be found, at chapter `3.3: Fixed-Point Facility Instructions`. We can see +here the instruction encoding, as well as its form (XO) and all information +we already found in `fixedarith.mdwn` file. + +All these details are available in some form as part of `info` variable we +observed before (at `caller.py` file). The `info` variable is, actually, +a simple instance of named tuple; the overall structure of this named tuple +is left as an exercise for the reader. + +# Modifying the pseudocode + +We won't dive into all gory details of the decoder, at least for now. What +is important for us at this stage is, how can we affect the generated code? +As we saw recently, the markdown file is somehow converted into the Python +code; but how is the conversion done? + +This is done by the script called pywriter. Again, we omit the exact details, +since this script clearly deserves its own documentation; the only crucial +information that pywriter script uses PLY (hint: Python-Lex-Yacc) in order +to get the work done, and, thanks to PLY magic, it is able to convert ISA +pseudocode into something more Pythonic. + +For illustrative purposes, let's modify the pseudocode so that, instead of +addition, it performs a subtraction. Let's find the pseudocode for `add` +instructions inside `fixedarith.mdwn`... ``` Pseudo-code: @@ -212,12 +257,18 @@ Pseudo-code: RT <- (RA) - (RB) ``` -Then re-run the unit test. Nothing happens (no change, test passes). then run -`pywriter noall fixedarith` and re-run the test, and it should **fail**. -Inspect logs, it shows a different result. +OK, we changed the pseudocode inside the Markdown, but it hasn't yet changed +a word inside the corresponding Python file (`fixedarith.py`). If we attempt +to re-run the unit test, nothing happens, since Python code is kept intact. + +In order to force the source code re-generation, `pywriter noall fixedarith` +command is used. Once `pywriter` completes its task, we can re-run the test, +and it should **fail**. The logs show a different result. Work out by hand what 0x1234 - 0x4321 in 64-bit unsigned arithmetic is, and -change the assert in the unit test to match. re-run: the test should now -**pass**. +change the assert in the unit test to match. Re-run: the test should now +**pass**. These manipulations, again, are left as an exercise for the reader. -TODO: expand. +This chapter concludes the First Steps tutorial. We'll take a deeper look +at pseudocode generation when we will dive into an example of modifying +BCD instructions.