[gcc r12-4400] Add FIXME note to backward threader.

[gcc r12-4400] Add FIXME note to backward threader.

[Aldy Hernandez]

https://gcc.gnu.org/g:27dea0b42c773a53817960d006160642892fe145

commit r12-4400-g27dea0b42c773a53817960d006160642892fe145
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Thu Oct 14 15:00:46 2021 +0200

    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.

Diff:
---
 gcc/tree-ssa-threadbackward.c | 7 +++++++
 1 file changed, 7 insertions(+)

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)
 	{