[gcc r17-1374] store-merging, riscv: Consider widen_bswap_or_bitreverse in store-merging decisions and fix up riscv

[Jakub Jelinek]

https://gcc.gnu.org/g:997a8a9bb85b615e9c1ec7fed802e203a6aaa4a3

commit r17-1374-g997a8a9bb85b615e9c1ec7fed802e203a6aaa4a3
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jun 5 17:16:59 2026 +0200

    store-merging, riscv: Consider widen_bswap_or_bitreverse in store-merging decisions and fix up riscv build
    
    On Fri, Jun 05, 2026 at 02:22:25PM +0200, Andreas Schwab wrote:
    > This breaks riscv:
    >
    > ../../gcc/config/riscv/riscv.md:5297:14: error: bswapsi2 cannot FAIL
    > ../../gcc/config/riscv/bitmanip.md:543:5: note: in expansion of macro 'FAIL'
    >   543 |     FAIL;
    >       |     ^~~~
    
    wtw mentioned that on IRC.
    In the way IFN_BSWAP/IFN_BITREVERSE are used currently (i.e. only used
    for large/huge _BitInt or in the C++ FE, but not making it to expansion)
    there is no reason why FAIL wouldn't be allowed (like before my changes),
    though I find the riscv hack really ugly.
    Instead of adding a workaround, this patch removes the riscv hack and
    instead adjusts store-merging pass to take into account
    widen_bswap_or_bitreverse; currently it already takes into account
    expand_doubleword_bswap_or_bitreverse (on 32-bit word targets which have
    bswapsi2 expanders expects __builtin_bswap64 to be cheap).  With this
    patch, it expects __builtin_bswap32 to be cheap if bswapdi2 expander
    is present, because widen_bswap_or_bitreverse will handle it as
    (subreg:SI (lshiftrt:DI (bswap:DI (subreg:DI (arg:SI) 0)) (const_int 32)) low)
    
    2026-06-05  Jakub Jelinek  <jakub@redhat.com>
    
            * gimple-ssa-store-merging.cc (maybe_optimize_vector_constructor):
            Also support bswap32 if bswapdi2 expander is present.  Add comment
            about bswap64 support on 32-bit targets with bswapsi2 expander.
            (pass_optimize_bswap::execute): Likewise.
            * config/riscv/bitmanip.md (bswapsi2): Change condition to disable
            the expander on TARGET_64BIT without TARGET_XTHEADBB, never FAIL.
    
    Reviewed-by: Richard Sandiford <rdsandiford@googlemail.com>
    Reviewed-by: Richard Biener <rguenth@suse.de>

Diff:
---
 gcc/config/riscv/bitmanip.md    | 11 +----------
 gcc/gimple-ssa-store-merging.cc | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index 05559405fcc9..bd2aaf0587eb 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -533,16 +533,7 @@
 (define_expand "bswapsi2"
   [(set (match_operand:SI 0 "register_operand")
 	(bswap:SI (match_operand:SI 1 "register_operand")))]
-  "TARGET_ZBB || TARGET_ZBKB || TARGET_XTHEADBB"
-{
-  /* Expose bswapsi2 on TARGET_64BIT so that the gimple store
-     merging pass will create suitable bswap insns.  We can actually
-     just FAIL that case when generating RTL and let the generic code
-     handle it.  */
-  if (TARGET_64BIT && !TARGET_XTHEADBB)
-    FAIL;
-})
-
+  "(!TARGET_64BIT && (TARGET_ZBB || TARGET_ZBKB)) || TARGET_XTHEADBB")
 
 (define_insn "*bswap<mode>2"
   [(set (match_operand:X 0 "register_operand" "=r")
diff --git a/gcc/gimple-ssa-store-merging.cc b/gcc/gimple-ssa-store-merging.cc
index 96fd9fa73a35..2e5232fa65ff 100644
--- a/gcc/gimple-ssa-store-merging.cc
+++ b/gcc/gimple-ssa-store-merging.cc
@@ -1570,7 +1570,10 @@ maybe_optimize_vector_constructor (gimple *cur_stmt)
       break;
     case 32:
       if (builtin_decl_explicit_p (BUILT_IN_BSWAP32)
-	  && optab_handler (bswap_optab, SImode) != CODE_FOR_nothing)
+	  && (optab_handler (bswap_optab, SImode) != CODE_FOR_nothing
+	      /* widen_bswap_or_bitreverse can implement 32-bit bswap
+		 using bswapdi2 and shift.  */
+	      || optab_handler (bswap_optab, DImode) != CODE_FOR_nothing))
 	{
 	  load_type = uint32_type_node;
 	  fndecl = builtin_decl_explicit (BUILT_IN_BSWAP32);
@@ -1582,6 +1585,8 @@ maybe_optimize_vector_constructor (gimple *cur_stmt)
     case 64:
       if (builtin_decl_explicit_p (BUILT_IN_BSWAP64)
 	  && (optab_handler (bswap_optab, DImode) != CODE_FOR_nothing
+	      /* expand_doubleword_bswap_or_bitreverse can use 2 bswapsi2
+		 expanders on 32-bit targets.  */
 	      || (word_mode == SImode
 		  && builtin_decl_explicit_p (BUILT_IN_BSWAP32)
 		  && optab_handler (bswap_optab, SImode) != CODE_FOR_nothing)))
@@ -1631,9 +1636,15 @@ pass_optimize_bswap::execute (function *fun)
   tree bswap32_type = NULL_TREE, bswap64_type = NULL_TREE;
 
   bswap32_p = (builtin_decl_explicit_p (BUILT_IN_BSWAP32)
-	       && optab_handler (bswap_optab, SImode) != CODE_FOR_nothing);
+	       && (optab_handler (bswap_optab, SImode) != CODE_FOR_nothing
+		   /* widen_bswap_or_bitreverse can implement 32-bit bswap
+		      using bswapdi2 and shift.  */
+		   || (optab_handler (bswap_optab, DImode)
+		       != CODE_FOR_nothing)));
   bswap64_p = (builtin_decl_explicit_p (BUILT_IN_BSWAP64)
 	       && (optab_handler (bswap_optab, DImode) != CODE_FOR_nothing
+		   /* expand_doubleword_bswap_or_bitreverse can use 2 bswapsi2
+		      expanders on 32-bit targets.  */
 		   || (bswap32_p && word_mode == SImode)));
 
   /* Determine the argument type of the builtins.  The code later on