CallArity: Consider shadowing introduced by case and field binders
In #20283 (closed), we saw a regression in simple
due to CallArity for a very subtle
reason: It simply didn't handle shadowing of case binders and constructor field
binders!
The test case T20283 has a very interesting binding n_X1
that we want to
eta-expand and that has a Unique (on GHC HEAD) that is reused by the Simplifier
for a case binder:
let { n_X1 = ... } in
...
let {
lvl_s1Ul
= ... case x_a1Rg of wild_X1 {
__DEFAULT -> f_s1Tx rho_value_awA (GHC.Types.I# wild_X1);
0# -> lvl_s1TN
} ...
} in
letrec {
go3_X3
= \ (x_X4 :: GHC.Prim.Int#) (v_a1P9 [OS=OneShot] :: Double) ->
let {
karg_s1Wu = ...
case lvl_s1Ul of { GHC.Types.D# y_a1Qf ->
...
} } in
case GHC.Prim.==# x_X4 y_a1R7 of {
__DEFAULT -> go3_X3 (GHC.Prim.+# x_X4 1#) karg_s1Wu;
1# -> n_X1 karg_s1Wu -- Here we will assume that karg calls n_X1!
}; } in
go3_X3 0#;
Since the Case case of CallArity doesn't delete X1
from the set of variables
it is interested in knowing the usages of, we leak a very boring usage (of the
case binder!) into the co-call graph that we mistakenly take for a usage of
n_X1
. We conclude that lvl_s1Ul
and transitively karg_s1Wu
call n_X1
when really they don't. That culminates in the conclusion that n_X1 karg_s1Wu
calls n_X1
more than once. Wrong!
Fortunately, this bug will never lead to a CallArity that is too optimistic. So by fixing this bug, we get strictly more opportunities for CallArity and all of them should be sound to exploit.
Fixes #20283 (closed).