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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/64154] enable fipa-ra for Thumb1
Date: Thu, 08 Jan 2015 13:55:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64154

--- Comment #3 from vries at gcc dot gnu.org ---
(In reply to Terry Guo from comment #2)
> Hi Tom,
> 
> I enabled this optimization to thumb1 target cortex-m0

Does that mean you just reverted the patch for PR63718?

> and found this IPA-RA
> optimization doesn't consider the register clobber information attached to
> call rtx

The IPA-RA analysis does consider that clobber. It's using
find_all_hard_reg_sets,  and that function contains a loop over
CALL_INSN_FUNCTION_USAGE.

> and thus generated bad code. Here are the bad rtxs I extracted from
> the dump of cprop_hardreg pass:
> 
> (insn 292 291 141 13 (set (reg:SI 4 r4 [orig:163 ivtmp.108 ] [163])
>         (reg:SI 12 ip [orig:163 ivtmp.108 ] [163])) 742 {*thumb1_movsi_insn}
>      (expr_list:REG_DEAD (reg:SI 12 ip [orig:163 ivtmp.108 ] [163])
>         (nil)))
> 
> (call_insn 141 292 142 13 (parallel [
>             (call (mem:SI (symbol_ref:SI ("f2") [flags 0x3]  <function_decl
> 0x7f8182689100 f2>) [0 f2 S4 A32])
>                 (const_int 0 [0]))
>             (use (const_int 0 [0]))
>             (clobber (reg:SI 14 lr))
>         ])
> /myssd/terguo01/toolchain-build/GCC32RM-424/src/gcc/gcc/testsuite/gcc.dg/
> vshift-3.c:119 770 {*call_insn}
>      (expr_list:REG_CALL_DECL (symbol_ref:SI ("f2") [flags 0x3] 
> <function_decl 0x7f8182689100 f2>)
>         (expr_list:REG_EH_REGION (const_int 0 [0])
>             (nil)))
>     (expr_list (clobber (reg:SI 12 ip))
>         (nil)))
> 
> (insn 11 10 12 13 (set (reg:SI 0 r0 [orig:170 ivtmp.130 ] [170])
>         (reg:SI 12 ip [orig:163 ivtmp.108 ] [163]))
> /myssd/terguo01/toolchain-build/GCC32RM-424/src/gcc/gcc/testsuite/gcc.dg/
> vshift-3.c:121 742 {*thumb1_movsi_insn}
>      (expr_list:REG_EQUAL (symbol_ref:SI ("j") [flags 0x80]  <var_decl
> 0x7f81827d0750 j>)
>         (nil)))
> 

That is wrong indeed: ip is explicitly clobbered by the call in
CALL_INSN_FUNCTION_USAGE, but used as if not clobbered by the call.

> I checked the code in 'if (CALL_P (insn))' part in file regcprop.c and found
> the algorithm doesn't consider the '(expr_list (clobber (reg:SI 12 ip))' in
> insn 141 which makes current algorithm think it is safe to propagate ip from
> insn 292 to insn 11.
> 

I cannot reproduce the wrong code as listed above. But I can reproduce the
clobber being skipped by copyprop_hardreg_forward_1. So I agree, cprop_hardreg
does not respect the clobber on the call in CALL_INSN_FUNCTION_USAGE.

I'll file a PR.