On 10/23/25 5:50 AM, Richard Biener wrote:
On Thu, 23 Oct 2025, Artemiy Volkov wrote:
This patch adds a match.pd transformation to strip unnecessary
view_converts of BIT_FIELD_REFs. This should work for all combinations of
types except where loss of precision is involved, i.e. in narrowing
conversions between integer types; this is checked for in the if clause of
the new pattern.
The change survives bootstrap + regtest on aarch64 and x86_64, and one
additional test case has been added to gcc.dg/tree-ssa.
gcc/ChangeLog:
* match.pd: Add pattern to simplify view_convert (BIT_FIELD_REF).
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/forwprop-42.c: New test.
---
gcc/match.pd | 8 ++++++++
gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c | 12 ++++++++++++
2 files changed, 20 insertions(+)
create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c
diff --git a/gcc/match.pd b/gcc/match.pd
index a4248a521cf..660f4ecb3cb 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -5624,6 +5624,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(view_convert (view_convert @0))
(view_convert @0))
+/* Squash view_converts of BFRs if no precision is lost. */
+(simplify
+ (view_convert (BIT_FIELD_REF@0 @1 @2 @3))
+ (if (!INTEGRAL_TYPE_P (type)
+ || !INTEGRAL_TYPE_P (TREE_TYPE (@0))
+ || TYPE_PRECISION (type) == TYPE_PRECISION (TREE_TYPE (@0)))
+ (BIT_FIELD_REF:type @1 @2 @3)))
I think you want to ensure that you do not fold in size changes.
See tree-cfg.cc:verify_types_in_gimple_reference. The view_convert
could convert to a variably-sized type.
I'm concerned there are many corner cases you can't think of. So
instead of a negative list (aka !INTEGRAL_TYPE_P) I'm looking for
a positive-list we are sure will work. I'd suggest to use
is_gimple_reg_type. Also require type_has_mode_precision_p
on integral types.
Just an FYI. I went down this path in a prior life. There's certainly
value in squashing out the useless V_C_E nodes. My recollection was it
simplified the match.pd/forwprop patterns we were writing to improve
extracting elements out of a vector. Sadly, never upstreamed and never
saw the light of day.
Point being it may not help much in isolation, but it can simplify
further work which obviously has value.
jeff
