Type checking and codegen still grow superlinearly after the #1995 fix

[nicolas-maman]

With the quadratic call-site propagation removed (#1995, PR #2006), the remaining compile time on a function-heavy file is split roughly evenly between type checking and codegen, and both still grow faster than linearly.

Measurement

One function per call site, best of 7 runs each. Phases separated with the compiler's own flags: --dump-ast is lex+parse, --check is through type checking, no flag is the whole compile.

functions	parse	type check	codegen	whole compile
400	0.016s	0.004s	0.008s	0.028s
800	0.019s	0.018s	0.024s	0.061s
1600	0.033s	0.072s	0.081s	0.185s

Growth per doubling of input:

phase	400 to 800	800 to 1600
parse	1.2x	1.7x
type check	4.5x	4.0x
codegen	3.0x	3.4x

Parse is linear. Type checking and codegen are not.

For contrast, the same table before #1995 was fixed: type check 0.028 / 0.090 / 0.326, whole compile 0.052 / 0.129 / 0.461. So #1995 removed the largest term, and 4.5x-per-doubling is what is left underneath it.

Why it is worth an issue rather than nothing

Same shape of argument as #1995. The 50000-token source cap currently bounds the input, so this is a scaling hazard rather than a live complaint, and the constants are small enough that the largest module in the tree (3472 lines) compiles in about 1.4s. Anyone who raises that cap, or lands a generated module, inherits the growth.

It is filed separately because it is two different passes with two different causes, and finding them starts with a profile rather than a code reading. #1995's cause was visible in the loop nesting; nothing here is.

Reproducing

n=1600
awk -v n=$n 'BEGIN{
  for(i=0;i<n;i++) printf "f%d(a) {\n    return a + %d;\n}\n", i, i;
  print "main() {"; print "    t = 0;";
  for(i=0;i<n;i++) printf "    t = t + f%d(%d);\n", i, i;
  print "    print(t);"; print "    print(\"\\n\");"; print "}" }' > /tmp/s.ae

build/aetherc --dump-ast /tmp/s.ae /tmp/out.c   # parse
build/aetherc --check    /tmp/s.ae /tmp/out.c   # through type check
build/aetherc            /tmp/s.ae /tmp/out.c   # whole compile

3200 is not measurable: the file hits the 50000-token cap first.

Found while verifying the fix for #1995.