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[COMMITTED] Add FIXME note to backward threader.

Message ID 20211014134251.237651-1-aldyh@redhat.com
State New
Headers show
Series [COMMITTED] Add FIXME note to backward threader. | expand

Commit Message

Aldy Hernandez Oct. 14, 2021, 1:42 p.m. UTC 
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/tree-ssa-threadbackward.c | 7 +++++++
 1 file changed, 7 insertions(+)
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Patch

diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 28c7ef8c872..496b68e0a82 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)
 	{