-fprofile-arcs overwriting return value of fn.

[Johannes Thoma <joe at mond dot at>]

Hi all,

I was experimenting with -fprofile-arcs on the new-reg-alloc branch. 
I found that on exit edges, it inserts insns for profilling after
the ``use'' of the register holding the return value which can
cause return values be overwritten: 

(insn 18 14 21 (set (reg/i:SI 0 eax)
        (reg:SI 58)) -1 (nil)
    (nil))

;; Instrumentation code should go here...
(insn 21 18 26 (use (reg/i:SI 0 eax)) -1 (nil)
    (nil))

;; This is inserted by -fprofile-arcs. 
(insn 26 21 28 (set (reg:SI 60)
        (symbol_ref:SI ("*.LPBX2"))) -1 (nil)
    (expr_list:REG_EQUAL (symbol_ref:SI ("*.LPBX2"))
        (nil)))

(insn 28 26 0 (parallel[
            (set (mem:DI (reg:SI 60) 0)
                (plus:DI (mem:DI (reg:SI 60) 0)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ] ) -1 (nil)
    (nil))
;; End of basic block 0, registers live:
 (nil)

Register allocator then assigns eax to pseudo reg 60 causing return
value be overwritten.

I wrote a quick patch that searches ``use'' insns back for all exit 
edges and inserts instrumentation code *before* the use insn .. 
(it doesn't bootstrap yet.), where to send it when it is ready? 

Or is it better to fix flow analysis (such that the ``use'' insn doesn't 
belong to basic block any more)?

Did anybody else observe the above? Is someone already working on 
a patch?

Thanks for any info (I am new to gcc mailing list and might ask
stupid questions ..)


- Johannes


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