On 7/20/2026 6:31 AM, Richard Biener wrote:
The following adds simplification of a scalar mode vec_select wrapping
a vec_concat. combine sees
(insn 7 4 8 2 (set (reg:V2DF 106)
(vec_concat:V2DF (plus:DF (vec_select:DF (reg/v:V2DF 101 [ p ])
(parallel [
(const_int 0 [0])
]))
(vec_select:DF (reg/v:V2DF 101 [ p ])
(parallel [
(const_int 1 [0x1])
])))
(plus:DF (vec_select:DF (reg/v:V2DF 101 [ p ])
(parallel [
(const_int 0 [0])
]))
(vec_select:DF (reg/v:V2DF 101 [ p ])
(parallel [
(const_int 1 [0x1])
]))))) "t.c":5:11 3234 {*sse3_haddv2df3}
(expr_list:REG_DEAD (reg/v:V2DF 101 [ p ])
(nil)))
(insn 8 7 9 2 (set (reg:DF 105)
(vec_select:DF (reg:V2DF 106)
(parallel [
(const_int 0 [0])
]))) "t.c":5:11 7297 {sse2_storelpd}
(expr_list:REG_DEAD (reg:V2DF 106)
(nil)))
and this should simplify and combine to
(set (reg:DF 105)
(plus:DF (vec_select:DF (reg:V2DF 109 [ p ])
(parallel [
(const_int 0 [0])
]))
(vec_select:DF (reg:V2DF 109 [ p ])
(parallel [
(const_int 1 [0x1])
]))))
which x86 can recognize (but rejects due to consting, see PR126328).
There'll be test coverage in gcc.target/i386/pr54400.c
Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
The code snipped is stripped down from the vector vec_select case
but with the side_effects_p arguments fixed(?)
OK?
Thanks,
Richard.
* simplify-rtx.cc (simplify_context::simplify_binary_operation_1):
Simplify scalar vec_select of vec_concat.
---
gcc/simplify-rtx.cc | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index 882a11c5760..612413bf035 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -5148,6 +5148,7 @@ simplify_ashift:
nested VEC_SELECT expressions. When input operand is a memory
operand, this operation can be simplified to a simple scalar
load from an offsetted memory address. */
+ int l0, l1;
int n_elts;
if (GET_CODE (trueop0) == VEC_SELECT
&& (GET_MODE_NUNITS (GET_MODE (XEXP (trueop0, 0)))
@@ -5210,6 +5211,23 @@ simplify_ashift:
tmp_op, gen_rtx_PARALLEL (VOIDmode, vec));
return tmp;
}
+ /* If we select one half of a vec_concat, return that. */
+ else if (GET_CODE (trueop0) == VEC_CONCAT
+ && (GET_MODE_NUNITS (GET_MODE (XEXP (trueop0, 0)))
+ .is_constant (&l0))
+ && (GET_MODE_NUNITS (GET_MODE (XEXP (trueop0, 1)))
+ .is_constant (&l1)))
You never use l1, though I guess what you're really testing is that it
is a constant, you don't need to know the precise constant?
Jeff
