Trivia: This code is older than many of the readers. https://github.com/mirrors/gcc/blame/7057506456ba18f080679b2...
1992.. older..
Now I feel old.
54-line if condition in gcc's reload.c
71–80 of 95 posts
Re: 54-line if condition in gcc's reload.c
#72Why put logical operator at the start and not the end of each line? I.e., this style (used in this case) && (CONSTANT_P (SUBREG_REG (in)) || GET_CODE (SUBREG_REG (in)) == PLUS || strict_low || (((REG_P (SUBREG_REG (in)) versus this style: (CONSTANT_P (SUBREG_REG (in)) || GET_CODE (SUBREG_REG (in)) == PLUS || strict_low || (((REG_P (SUBREG_REG (in)) && I don't have a personal preference here, just looking for any prac…
It’s from the GNU Coding Standards, section 5.1 Formatting Your Source Code¹:
When you split an expression into multiple lines, split it before an operator, not after one. Here is the right way:
if (foo_this_is_long && bar > win (x, y, z)
&& remaining_condition)
Re: 54-line if condition in gcc's reload.c
#73Trivia: This code is older than many of the readers. https://github.com/mirrors/gcc/blame/7057506456ba18f080679b2...
That's beautiful. It may be a monster, but it started so small in 1992 - then was grown with loving care in 1993, 1994, 1995, 1998, 1999, 2001, 2002, 2004, 2005, 2008, and 2011!
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Re: 54-line if condition in gcc's reload.c
#74The rest of that file also has a ton of redundancies and verboseness its in logic that could be simplified considerably; e.g. I see this pattern a lot (428 ~ 435): x && y || !x && z In this absence of side-effects, this basically implements a 2-input multiplexer and is identical to x ? y : z In the 54-line condition the first obvious thing I'd factor out is SUBREG_REG(in) and GET_MODE(SUBREG_REG(in)), and then work o…
The way it was for me was that I only "got" the hang of the ternary operator once I learned what a multiplexer is and how it's implemented. Before that, it was something I used now and then (certainly didn't have any trouble understanding it or anything like that), but I never thought about it as some way to channel data through based on a condition. I just always thought of it as some handy form of conditional evaluation. Things "clicked" a lot when I learned about how digital hardware and muxes works.
Re: 54-line if condition in gcc's reload.c
#75Earlier quoted context omitted.
I used to use the latter, in both code and maths, but I have switched to the former. My main reason is that having the operators (logical and otherwise) at the start of the line means they're more likely to be in the same horizontal position, so it's easier to see which lines are continuations of the previous ones.
It is also easier to comment out lines of code if you need to test something. This is especially useful in SQL, where you can quickly -- a condition or a column, without editing commas etc on other lines, eg SELECT some_col , another_col --, and_another , and_more FROM blah EDIT: too bad formatting is screwed up :/
Isn't this behaviour exactly the same as with commas at the end? Here, to comment out the first line you would need to edit the second. With commas at the end, to comment out the last line you would need to edit the second last. Other than that, no editing of other lines is required.
In fact, in a language like Javascript where extra trailing commas are allowed, it seems that this argument makes even more sense for commas at the end than it does for commas at the beginning. Then, there would never be a situation where you would have to edit a line besides the one you were commenting out.
Re: 54-line if condition in gcc's reload.c
#76Why put logical operator at the start and not the end of each line? I.e., this style (used in this case) && (CONSTANT_P (SUBREG_REG (in)) || GET_CODE (SUBREG_REG (in)) == PLUS || strict_low || (((REG_P (SUBREG_REG (in)) versus this style: (CONSTANT_P (SUBREG_REG (in)) || GET_CODE (SUBREG_REG (in)) == PLUS || strict_low || (((REG_P (SUBREG_REG (in)) && I don't have a personal preference here, just looking for any prac…
I like the former, since it's more readable. I mean as in written English, you should put conjunctions at the start of a new line instead of letting them "hang". So you get
constant...
or get code...
or strict low
instead of
constant... or
get code... or
strict low
Re: 54-line if condition in gcc's reload.c
#77For the curious, reload was written by Richard Kenner, who, while a wonderful guy, was not generally familiar with modern compiler architecture (graph coloring register allocation goes back to 1982, reload was started in the late 80's). It essentially took the place of spill placement/code/legalization. Over the years, it grew rematerialization, instruction combination, copy coalescing, stack slot sharing and all sor…
As someone moderately curious, I hope I'm not the only one when I say: I didn't understand a word after "the late 80's". And that's only because I searched for "graph coloring register allocation" first.
For simplicity, a compiler manipulates an internal representation in terms of virtual registers. A register allocator assigns physical registers to these virtual ones, under the principle that different virtual registers may be assigned to the same physical one if they are not live (I.e. hold a value that will be used in the future) at the same time.
Spill code generation is necessary because at a program point, there may not be enough physical registers for all the live values, and code must be generated to spill some of them to the stack and reload them when needed. Legalization performs code transformations to eliminate target independent operations in the IR that can't be represented on the target. Copy coalescing eliminates the need for copies between virtual registers by assigning both to the same physical register. Rematerialization takes advantage of values that can be recomputed cheaply, and instead of holding them in a register for a long time, recomputes them where needed.
What makes this all so complex is that 1) most of these don't have polynomial time algorithms that produce optimal solutions: 2) the individual problems are mostly coupled. E.g. spill code itself uses registers, and so the interference graph (which summaries whether virtual registers are simultaneously live) might need to be recomputed or modified.
If you're interested in this sort of thing, Kieth Cooper at Rice has posted the lecture notes for his graduate compilers class online: http://www.cs.rice.edu/~keith/512/Lectures. Register allocation is lectures 26-7.
Re: 54-line if condition in gcc's reload.c
#78Needs more goto's.
Re: 54-line if condition in gcc's reload.c
#79Earlier quoted context omitted.
It is also easier to comment out lines of code if you need to test something. This is especially useful in SQL, where you can quickly -- a condition or a column, without editing commas etc on other lines, eg SELECT some_col , another_col --, and_another , and_more FROM blah EDIT: too bad formatting is screwed up :/
Isn't this behaviour exactly the same as with commas at the end? Here, to comment out the first line you would need to edit the second. With commas at the end, to comment out the last line you would need to edit the second last. Other than that, no editing of other lines is required. In fact, in a language like Javascript where extra trailing commas are allowed, it seems that this argument makes even more sense for c…
Not if the line you're commenting out is the last item of a select or a join command or an AND within a where clause. Sorry Gould have made better example
But it was 4 am :)
Re: 54-line if condition in gcc's reload.c
#80Trivia: This code is older than many of the readers. https://github.com/mirrors/gcc/blame/7057506456ba18f080679b2...
By 1992 I had already graduated...
Great. Now I feel a million years old...