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| author | Aldy Hernandez <aldyh@redhat.com> | 2021-10-14 15:00:46 +0200 |
|---|---|---|
| committer | Aldy Hernandez <aldyh@redhat.com> | 2021-10-14 15:43:41 +0200 |
| commit | 27dea0b42c773a53817960d006160642892fe145 (patch) | |
| tree | f6cc4ef30055858562d1725a793b99b4e41b3aa2 | |
| parent | ebdf180e15d0ae18bfcb2bd822d7f096743cd4fb (diff) | |
| download | gcc-27dea0b42c773a53817960d006160642892fe145.zip gcc-27dea0b42c773a53817960d006160642892fe145.tar.gz gcc-27dea0b42c773a53817960d006160642892fe145.tar.bz2 | |
Add FIXME note to backward threader.
There's a limitation in the path discovery bits in the backward
threader that I ran into recently and I'd like to document it so we
don't lose track of it.
Basically we stop looking if we find a threadable path through a PHI,
without taking into account that there could be multiple
paths through a PHI that resolve the path. For example:
x_5 = PHI <10(4), 20(5), ...>
if (x_5 > 5)
I don't remember how we ended up skipping this, but it could existing
behavior from the old bits. It probably skipped multiple threads
through a PHI since the generic copier couldn't re-using existing
threading paths anyhow.
Documenting for later fixing.
gcc/ChangeLog:
* tree-ssa-threadbackward.c (back_threader::resolve_phi): Add
FIXME note.
| -rw-r--r-- | gcc/tree-ssa-threadbackward.c | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c index 28c7ef8..496b68e 100644 --- a/gcc/tree-ssa-threadbackward.c +++ b/gcc/tree-ssa-threadbackward.c @@ -282,6 +282,13 @@ back_threader::resolve_phi (gphi *phi, bitmap interesting) continue; } + // FIXME: We currently stop looking if we find a threadable path + // through a PHI. This is pessimistic, as there can be multiple + // paths that can resolve the path. For example: + // + // x_5 = PHI <10(4), 20(5), ...> + // if (x_5 > 5) + tree arg = gimple_phi_arg_def (phi, i); if (TREE_CODE (arg) == SSA_NAME) { |