On 32-bit sparc with LRA enabled we have the following (this
generated for gcc.dg/vect/pr51581-4.c with -flto):
(insn 252 142 165 4 (set (reg:HI 234 [ D.1511 ])
(mem/c:HI (plus:SI (reg/f:SI 1307)
(const_int 24 [0x18])) [4 b+24 S2 A64])) test.c:23 59 {*movhi_insn}
(expr_list:REG_EQUIV (mem/c:HI (plus:SI (reg/f:SI 1307)
(const_int 24 [0x18])) [4 b+24 S2 A64])
(expr_list:REG_EQUAL (mem/c:HI (const:SI (plus:SI (symbol_ref:SI ("b.1425") [flags 0x2] <\
var_decl 0xf7519740 b>)
(const_int 24 [0x18]))) [4 b+24 S2 A64])
(nil))))
...
(insn 255 221 233 4 (set (reg:SI 1285)
(ashift:SI (subreg:SI (reg:HI 234 [ D.1511 ]) 0)
(const_int 16 [0x10]))) test.c:23 306 {ashlsi3}
(expr_list:REG_DEAD (reg:HI 234 [ D.1511 ])
(nil)))
When LRA does reloads for insn 255, it tries to strip the subregs
for pseudo 234 and use SImode moves.
(insn 1367 1732 1712 4 (set (reg:SI 12 %o4 [1425])
(mem/c:SI (plus:SI (reg/f:SI 3 %g3 [1307])
(const_int 22 [0x16])) [4 b+22 S4 A16])) test.c:23 61 {*movsi_insn}
(nil))
(insn 255 1712 1366 4 (set (reg:SI 12 %o4 [1285])
(ashift:SI (reg:SI 12 %o4 [1425])
(const_int 16 [0x10]))) test.c:23 306 {ashlsi3}
(expr_list:REG_DEAD (reg:SI 12 %o4 [1425])
(nil)))
(insn 1366 255 1711 4 (set (reg:SI 12 %o4 [1285])
(reg:SI 12 %o4 [1285])) test.c:23 61 {*movsi_insn}
(expr_list:REG_DEAD (reg:SI 12 %o4 [1285])
(nil)))
Unfortunately, this is illegal, because insn 1367 is an SImode
load to a non-4-byte-aligned location (%g3 + 22).
This does work, and might even be profitable, on non-STRICT_ALIGNMENT
targets. But for those which are STRICT_ALIGNMENT, we cannot make
transformations like this.