[gcc r12-3482] gimple: allow more folding of memcpy [PR102125]

[Richard Earnshaw]

https://gcc.gnu.org/g:5f6a6c91d7c592cb49f7c519f289777eac09bb74

commit r12-3482-g5f6a6c91d7c592cb49f7c519f289777eac09bb74
Author: Richard Earnshaw <rearnsha@arm.com>
Date:   Fri Sep 3 17:06:15 2021 +0100

    gimple: allow more folding of memcpy [PR102125]
    
    The current restriction on folding memcpy to a single element of size
    MOVE_MAX is excessively cautious on most machines and limits some
    significant further optimizations.  So relax the restriction provided
    the copy size does not exceed MOVE_MAX * MOVE_RATIO and that a SET
    insn exists for moving the value into machine registers.
    
    Note that there were already checks in place for having misaligned
    move operations when one or more of the operands were unaligned.
    
    On Arm this now permits optimizing
    
    uint64_t bar64(const uint8_t *rData1)
    {
        uint64_t buffer;
        memcpy(&buffer, rData1, sizeof(buffer));
        return buffer;
    }
    
    from
            ldr     r2, [r0]        @ unaligned
            sub     sp, sp, #8
            ldr     r3, [r0, #4]    @ unaligned
            strd    r2, [sp]
            ldrd    r0, [sp]
            add     sp, sp, #8
    
    to
            mov     r3, r0
            ldr     r0, [r0]        @ unaligned
            ldr     r1, [r3, #4]    @ unaligned
    
    PR target/102125 - (ARM Cortex-M3 and newer) missed optimization. memcpy not needed operations
    
    gcc/ChangeLog:
    
            PR target/102125
            * gimple-fold.c (gimple_fold_builtin_memory_op): Allow folding
            memcpy if the size is not more than MOVE_MAX * MOVE_RATIO.

Diff:
---
 gcc/gimple-fold.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
index dd0e6b5daff..6fea8a6f9fd 100644
--- a/gcc/gimple-fold.c
+++ b/gcc/gimple-fold.c
@@ -67,6 +67,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-vector-builder.h"
 #include "tree-ssa-strlen.h"
 #include "varasm.h"
+#include "memmodel.h"
+#include "optabs.h"
 
 enum strlen_range_kind {
   /* Compute the exact constant string length.  */
@@ -957,14 +959,17 @@ gimple_fold_builtin_memory_op (gimple_stmt_iterator *gsi,
 	= build_int_cst (build_pointer_type_for_mode (char_type_node,
 						      ptr_mode, true), 0);
 
-      /* If we can perform the copy efficiently with first doing all loads
-         and then all stores inline it that way.  Currently efficiently
-	 means that we can load all the memory into a single integer
-	 register which is what MOVE_MAX gives us.  */
+      /* If we can perform the copy efficiently with first doing all loads and
+	 then all stores inline it that way.  Currently efficiently means that
+	 we can load all the memory with a single set operation and that the
+	 total size is less than MOVE_MAX * MOVE_RATIO.  */
       src_align = get_pointer_alignment (src);
       dest_align = get_pointer_alignment (dest);
       if (tree_fits_uhwi_p (len)
-	  && compare_tree_int (len, MOVE_MAX) <= 0
+	  && (compare_tree_int
+	      (len, (MOVE_MAX
+		     * MOVE_RATIO (optimize_function_for_size_p (cfun))))
+	      <= 0)
 	  /* FIXME: Don't transform copies from strings with known length.
 	     Until GCC 9 this prevented a case in gcc.dg/strlenopt-8.c
 	     from being handled, and the case was XFAILed for that reason.
@@ -1000,6 +1005,7 @@ gimple_fold_builtin_memory_op (gimple_stmt_iterator *gsi,
 	      if (type
 		  && is_a <scalar_int_mode> (TYPE_MODE (type), &mode)
 		  && GET_MODE_SIZE (mode) * BITS_PER_UNIT == ilen * 8
+		  && have_insn_for (SET, mode)
 		  /* If the destination pointer is not aligned we must be able
 		     to emit an unaligned store.  */
 		  && (dest_align >= GET_MODE_ALIGNMENT (mode)