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[Tinycc-devel] Jump optimization questions - commit: 8227db3a

From: Ekaitz Zarraga
Subject: [Tinycc-devel] Jump optimization questions - commit: 8227db3a
Date: Tue, 09 Aug 2022 18:39:57 +0000

Hi all,

I'm working on the RISC-V bootstrapping efforts for Guix, and I have to 
backport the RISC-V backend to an older TinyCC version we have patched in order 
to be able to build it with a simpler compiler.

The process worked mostly ok, but I can't understand very well the `gtst` 
function in the generation part, and I need to write it from scratch as the 
commit 8227db3a changed how the tests and jumps are managed.

I read the `gtst` function in other architectures but I don't understand every 
field, I hope you can help me.

This is the code I managed to arrange, following what I can see in the arm and 
i386 targets, but I need to fill the gaps. I added some comments in the hopes 
you can clarify them to me.

The thing that makes me be lost the most is the variable names... `t` and such 
are a little bit difficult to follow.

``` c
/* generate a test. set 'inv' to invert test. Stack entry is popped */
int gtst(int inv, int t)
  // I'd love be sure of what's `t` -> I think it's the jump target. In all the
  // gtst calls I see it's `0` but then when other things are called it's set
  // to another value and returned, so it might be that: the test doesn't need
  // to jump until we insert instructions after it...?
  int v = vtop->r & VT_VALMASK;
  int r=ind;
  // What's `ind`? the current location in the code (it looks like because
  // it's incremented in o() )

  if (nocode_wanted) {
  } else if (v == VT_CMP) {
      /* TODO */
       * vtop->c.i  contains the kind of the operation we need to do (we need
       * to invert it if we have `inv` set, we can use `negcc` for that, which
       * just converts the value to the negated version )
       *  - TOK_ULT
       *  - TOK_UGE
       *  - TOK_EQ
       *  - TOK_NE
       *  - TOK_ULE
       *  - TOK_UGT
       *  - TOK_Nset
       *  - TOK_Nclear
       *  - TOK_LT
       *  - TOK_GE
       *  - TOK_LE
       *  - TOK_GT
       *  We generate the operation with that, and we need to insert the branch
       *  too: then o(operation) to generate the code.
       *  The branch is obtained from:
       *  - r -> position
       *  - t -> address
       *  - fail -> for the case that the jump is too large to be encoded here
       *     `- That should load the address in steps and all that, but arm
       *        doesn't implement that yet, so we won't
       *  RISC-V does not have CPU flags, where's the value stored in that case?
  } else if (v == VT_JMP || v == VT_JMPI) {
      // && and || optimization I don't understand
      /* Check if jump is inverted (v&1) and if the gtst was inverted at the 
same time */
      if ((v & 1) == inv) {
              // ??
              // DOES SOME INSTRUCTION DECODING HERE -> patching previous
              // jumps?
      } else {
          // Just jump?
          // Why return `t`?
          t = gjmp(t);
  return t;

Thank you all for the help,

PS: grischka, I added you on copy as you are the autor of the mentioned commit.

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