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RISC-V: Skip vector adjustments for scalar costing

Message ID 20260902175149.70623-1-wangyaduo@linux.alibaba.com
State New
Headers
Series RISC-V: Skip vector adjustments for scalar costing |

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Commit Message

Wang Yaduo Sept. 2, 2026, 5:51 p.m. UTC 
  Recovered scalar types are needed by the base cost hook, but they must not
trigger vector-specific statement cost adjustments.

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (costs::add_stmt_cost): Skip
	statement cost adjustments for scalar costing.
---
When costing scalar IL, vectype is null and 32dc4bc recovers a type from
the statement LHS.  The LHS can itself be vector-typed, however, so this
can produce a vectype that is wrong for the scalar cost being computed.

This happens for the 512-element constructor in combine-4.c.  Its root
statement has a vector(512) long int LHS.  Recovering that type while
computing SCALAR_COST lets adjust_stmt_cost apply M8 scaling, raising the
scalar cost from 1 to 8.  An SLP alternative using 512-byte vectors, whose
cost is 6, then appears profitable and expands the constructor through a
4096-byte stack temporary.  This removes one of the four expected
vslideup.vx instructions.

Require m_cost_type != SCALAR_COST before calling adjust_stmt_cost.  The
recovered type is still passed to builtin_vectorization_cost, but cannot
trigger vector-specific adjustments during scalar costing.

Tested the full gcc.target/riscv testsuite on rv64gcv, including execution
tests; no new FAIL, XPASS, UNRESOLVED, or ERROR results versus 32dc4bc^.

 gcc/config/riscv/riscv-vector-costs.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/gcc/config/riscv/riscv-vector-costs.cc b/gcc/config/riscv/riscv-vector-costs.cc
index dfb80bee67..41a15ae220 100644
--- a/gcc/config/riscv/riscv-vector-costs.cc
+++ b/gcc/config/riscv/riscv-vector-costs.cc
@@ -1622,7 +1622,7 @@  costs::add_stmt_cost (int count, vect_cost_for_stmt kind,
 	m_unrolled_vls_stmts += count * m_unrolled_vls_niters;
     }
 
-  if (vectype)
+  if (m_cost_type != SCALAR_COST && vectype)
     stmt_cost = adjust_stmt_cost (kind, loop_vinfo, stmt_info, node, vectype,
 				  stmt_cost);