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From: "guihaoc at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/95737] PPC: Unnecessary extsw after negative less than
Date: Fri, 14 Jan 2022 09:16:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95737

--- Comment #9 from HaoChen Gui <guihaoc at gcc dot gnu.org> ---
Add a pattern to convert the plus mode to DI.=20

+(define_insn_and_split "*my_split"
+  [(set (match_operand:DI 0 "gpc_reg_operand")
+       (sign_extend:DI (plus:SI (match_operand:SI 1 "ca_operand")
+                                (const_int -1))))]
+  ""
+  "#"
+  ""
+  [(parallel [(set (match_dup 0)
+                  (plus:DI (match_dup 2)
+                           (const_int -1)))
+             (clobber (match_dup 2))])]
+{
+  operands[2] =3D copy_rtx (operands[1]);
+  PUT_MODE (operands[2], DImode);
+})

With the patch, the "extsw" could be optimized out. I compared the performa=
nce
between P8 code (with the patch) and P9 code. The performance of P9 is bett=
er.=20
ISA says that computation with CA causes additional latency. It should be t=
rue.
The only concern is P9 code uses more register.=