[PATCH, RTL opt]: Ensure that equivalent_reg_at_start rejects registers with mismatched mode

[PATCH, RTL opt]: Ensure that equivalent_reg_at_start rejects registers with mismatched mode

[Uros Bizjak]

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Hello!

-mtune=intel is able to copy SFmode value through general registers,
resulting in the following sequence:

(insn 288 1008 1009 29 (parallel [
            (set (reg:DI 4 si [687])
                (lshiftrt:DI (reg:DI 4 si [687])
                    (const_int 32 [0x20])))
            (clobber (reg:CC 17 flags))
        ]) "ttt.c":41 546 {*lshrdi3_1}
     (nil))
(insn 1009 288 289 29 (set (reg:DI 2 cx [419])
        (reg:DI 4 si [687])) "ttt.c":41 81 {*movdi_internal}
     (nil))
(insn 289 1009 291 29 (set (reg:SF 21 xmm0 [425])
        (reg:SF 2 cx [419])) "ttt.c":41 127 {*movsf_internal}
     (nil))
(insn 291 289 292 29 (set (reg:CCFPU 17 flags)
        (compare:CCFPU (reg:SF 21 xmm0 [425])
            (reg:SF 27 xmm6 [688]))) "ttt.c":41 51 {*cmpiusf}
     (expr_list:REG_EQUAL (compare:CCFPU (reg:SF 21 xmm0 [425])
            (const_double:SF 0.0 [0x0.0p+0]))
        (nil)))

Looking at compare-elim.c, equivalent_reg_at_start, the function
traces the value in %xmm0 to %rsi. So, try_eliminate_compare tries to
check if the target supports the compare of

                 (lshiftrt:DI (reg:DI 4 si [687])
                    (const_int 32 [0x20])))

with

(reg:SF 27 xmm6 [688])

which won't fly, since compare operands must have the same modes.

Attached patch ensures that the mode of register copy, found by
equivalent_reg_at_start equals original mode.

2017-05-13  Uros Bizjak  <ubizjak@gmail.com>

    * compare-elim.c (equivalent_reg_at_start): Return NULL_RTX
    when returned register mode doesn't match original mode.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Committed to mainline SVN as obvious.

Uros.

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Index: compare-elim.c
===================================================================
--- compare-elim.c	(revision 248008)
+++ compare-elim.c	(working copy)
@@ -526,6 +526,7 @@ maybe_select_cc_mode (struct comparison *cmp, rtx
 static rtx
 equivalent_reg_at_start (rtx reg, rtx_insn *end, rtx_insn *start)
 {
+  machine_mode orig_mode = GET_MODE (reg);
   rtx_insn *bb_head = BB_HEAD (BLOCK_FOR_INSN (end));
 
   for (rtx_insn *insn = PREV_INSN (end);
@@ -572,6 +573,9 @@ equivalent_reg_at_start (rtx reg, rtx_insn *end, r
 	return NULL_RTX;
     }
 
+  if (GET_MODE (reg) != orig_mode)
+    return NULL_RTX;
+
   return reg;
 }