From: Robin Dapp <rdapp@ventanamicro.com>
Date: Thu, 6 Nov 2025 08:14:35 +0000 (+0100)
Subject: vect: Do not convert offset type in strided gather.
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=c07453dffe3c2dafd4c968a9f79c726ab816ca4b;p=thirdparty%2Fgcc.git

vect: Do not convert offset type in strided gather.

The gather/scatter relaxation patches introduced a bug with
vect_use_strided_gather_scatters_p.  I didn't want to pass
supported_offset_vectype and supported scale all the way from
vect_truncate_gather_scatter_offset and
vect_use_strided_gather_scatters_p to get_load_store_type so
just called vect_gather_scatter_fn_p again afterwards to determine
the supported type and scale.

However, this doesn't take into account that
vect_use_strided_gather_scatters_p changes the offset type after
verifying that we can use gather/scatter.

The flow right now is
 - vect_use_strided_gather_scatters_p calls vect_check_gather_scatter
 with e.g. a char offset type.
 - We actually need/support a short vector offset type and
 vect_use_strided_gather_scatters_p fold converts the actual (scalar)
 char offset to a short offset.
 - We call vect_gather_scatter_fn_p with the new short offset instead of
 the original char one, thinking we need an even larger offset type.

The last call is obviously not identical to the ones we used to check
gather/scatter in the first place and can fail if there is no offset
vectype.

There are several ways to fix this.  The most obvious one is to bite the
bullet and just add the supported_offset_vectype and supported_scale to
all the intermediate functions.  I wondered, however, if we need the
offset conversion at all.  As far as I can tell we don't ever use
the scalar offset type and vect_get_strided_load_store_ops in particular
uses offset_vectype.  This, this patch removes the conversion.

I bootstrapped and regtested this, before and after the relaxation
patches, on x86 and power10.  Regtested on aarch64 and riscv.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vect_use_strided_gather_scatters_p):
	Do not convert offset type.
---

diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index 2054f2afa6e..1620fc6cbcc 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -1856,15 +1856,6 @@ vect_use_strided_gather_scatters_p (stmt_vec_info stmt_info, tree vectype,
 						masked_p, gs_info, elsvals))
 	return false;
     }
-  else
-    {
-      tree old_offset_type = TREE_TYPE (gs_info->offset);
-      tree new_offset_type = TREE_TYPE (gs_info->offset_vectype);
-
-      gcc_assert (TYPE_PRECISION (new_offset_type)
-		  >= TYPE_PRECISION (old_offset_type));
-      gs_info->offset = fold_convert (new_offset_type, gs_info->offset);
-    }
 
   if (!single_element_p
       && !targetm.vectorize.prefer_gather_scatter (TYPE_MODE (vectype),