As outlined in the PR, we end up generating poor code because of the
existence of the ashldi_extsi insn in the m68k backend.
The pattern recognizes that left shifting a DImode value by 32 deposits
the low part of the input into the high part of the output and clears
the low part of the output.
That's a fine thing to recognize, except that it's a two instruction
insn. If (for example) the next insn happens to overwrite those low
bits then the clearing done by the 2nd instruction generated by
ashldi_extsi is dead.
The obvious way to fix this is to turn the pattern into a
define_insn_and_split. One could certainly make an argument that to the
extent possible every insn that generates multiple instructions like
this ought to turn into a define_insn_and_split. However, I don't think
there's enough interest in the m68k port to warrant the time spent.
Tested by building m68k cross and verifying correct output for the
testcase. Installed on the trunk.
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index ce9c066..1237904 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,9 @@
2014-02-07 Jeff Law <l...@redhat.com>
+ PR target/40977
+ * config/m68k/m68k.md (ashldi_extsi): Turn into a
+ define_insn_and_split.
+
* ipa-inline.c (inline_small_functions): Fix typos.
2014-02-07 Richard Sandiford <rsand...@linux.vnet.ibm.com>
diff --git a/gcc/config/m68k/m68k.md b/gcc/config/m68k/m68k.md
index 7bf9abd..e61048b 100644
--- a/gcc/config/m68k/m68k.md
+++ b/gcc/config/m68k/m68k.md
@@ -4338,25 +4338,18 @@
;; arithmetic shift instructions
;; We don't need the shift memory by 1 bit instruction
-
-(define_insn "ashldi_extsi"
+(define_insn_and_split "ashldi_extsi"
[(set (match_operand:DI 0 "nonimmediate_operand" "=ro")
(ashift:DI
(match_operator:DI 2 "extend_operator"
[(match_operand:SI 1 "general_operand" "rm")])
(const_int 32)))]
""
-{
- CC_STATUS_INIT;
- if (GET_CODE (operands[0]) == REG)
- operands[2] = gen_rtx_REG (SImode, REGNO (operands[0]) + 1);
- else
- operands[2] = adjust_address (operands[0], SImode, 4);
- if (ADDRESS_REG_P (operands[0]))
- return "move%.l %1,%0\;sub%.l %2,%2";
- else
- return "move%.l %1,%0\;clr%.l %2";
-})
+ "#"
+ "&& reload_completed"
+ [(set (match_dup 3) (match_dup 1))
+ (set (match_dup 2) (const_int 0))]
+ "split_di(operands, 1, operands + 2, operands + 3);")
(define_insn "ashldi_sexthi"
[(set (match_operand:DI 0 "nonimmediate_operand" "=m,a*d")
