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Add FIXME note to backward threader.

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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.
This commit is contained in:
Aldy Hernandez 2021-10-14 15:00:46 +02:00
parent ebdf180e15
commit 27dea0b42c
1 changed files with 7 additions and 0 deletions
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7
gcc/tree-ssa-threadbackward.c
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@ -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)
{