Re: PATCH: PR rtl-optimization/64037: Miscompilation with -Os and enum class : char parameter

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Nov 25, 2014 at 4:01 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Nov 25, 2014 at 1:57 PM, H.J. Lu <hongjiu.lu@intel.com> wrote:
>> Hi,
>>
>> The enclosed testcase fails on x86 when compiled with -Os since we pass
>> a byte parameter with a byte load in caller and read it as an int in
>> callee.  The reason it only shows up with -Os is x86 backend encodes
>> a byte load with an int load if -O isn't used.  When a byte load is
>> used, the upper 24 bits of the register have random value for none
>> WORD_REGISTER_OPERATIONS targets.
>>
>> It happens because setup_incoming_promotions in combine.c has
>>
>>       /* The mode and signedness of the argument before any promotions happen
>>          (equal to the mode of the pseudo holding it at that stage).  */
>>       mode1 = TYPE_MODE (TREE_TYPE (arg));
>>       uns1 = TYPE_UNSIGNED (TREE_TYPE (arg));
>>
>>       /* The mode and signedness of the argument after any source language and
>>          TARGET_PROMOTE_PROTOTYPES-driven promotions.  */
>>       mode2 = TYPE_MODE (DECL_ARG_TYPE (arg));
>>       uns3 = TYPE_UNSIGNED (DECL_ARG_TYPE (arg));
>>
>>       /* The mode and signedness of the argument as it is actually passed,
>>          after any TARGET_PROMOTE_FUNCTION_ARGS-driven ABI promotions.  */
>>       mode3 = promote_function_mode (DECL_ARG_TYPE (arg), mode2, &uns3,
>>                                      TREE_TYPE (cfun->decl), 0);
>>
>> while they are actually passed in register by assign_parm_setup_reg in
>> function.c:
>>
>>   /* Store the parm in a pseudoregister during the function, but we may
>>      need to do it in a wider mode.  Using 2 here makes the result
>>      consistent with promote_decl_mode and thus expand_expr_real_1.  */
>>   promoted_nominal_mode
>>     = promote_function_mode (data->nominal_type, data->nominal_mode, &unsignedp,
>>                              TREE_TYPE (current_function_decl), 2);
>>
>> where nominal_type and nominal_mode are set up with TREE_TYPE (parm)
>> and TYPE_MODE (nominal_type). TREE_TYPE here is
>
> I think the bug is here, not in combine.c.  Can you try going back in history
> for both snippets and see if they matched at some point?

Oh, and note that I think DECL_ARG_TYPE is sth dangerous - it's meant
to be a source language "ABI" kind-of-thing.  Or rather an optimization
hit.  For example in C when integral promotions happen to call arguments
this can be used to optimize sign-/zero-extensions in the callee.  Unless
something else overrides this (like the target which specifies the real ABI).
IIRC.

Richard.

> Thanks,
> Richard.
>
>> (gdb) call debug_tree (type)
>>  <enumeral_type 0x7ffff19f85e8 X
>>     type <integer_type 0x7ffff18a93f0 unsigned char public unsigned
>> string-flag QI
>>         size <integer_cst 0x7ffff18a5fa8 constant 8>
>>         unit size <integer_cst 0x7ffff18a5fc0 constant 1>
>>         align 8 symtab 0 alias set -1 canonical type 0x7ffff18a93f0
>> precision 8 min <integer_cst 0x7ffff18a5fd8 0> max <integer_cst
>> 0x7ffff18a5f78 255>>
>>     static unsigned type_5 QI size <integer_cst 0x7ffff18a5fa8 8> unit
>> size <integer_cst 0x7ffff18a5fc0 1>
>>     align 8 symtab 0 alias set -1 canonical type 0x7ffff19f85e8
>> precision 8 min <integer_cst 0x7ffff18a5fd8 0> max <integer_cst
>> 0x7ffff18a5f78 255>
>>     values <tree_list 0x7ffff19fb028
>>         purpose <identifier_node 0x7ffff19f6738 V
>>             bindings <(nil)>
>>             local bindings <(nil)>>
>>         value <const_decl 0x7ffff18c21c0 V type <enumeral_type
>> 0x7ffff19f85e8 X>
>>             readonly constant used VOID file pr64037.ii line 2 col 3
>>             align 1 context <enumeral_type 0x7ffff19f85e8 X> initial
>> <integer_cst 0x7ffff19d8d08 2>>> context <translation_unit_decl
>> 0x7ffff7ff91e0 D.1>
>>     chain <type_decl 0x7ffff19f5c78 X>>
>> (gdb)
>>
>> and DECL_ARG_TYPE is
>>
>> (gdb) call debug_tree (type)
>>  <integer_type 0x7ffff18a9690 int public SI
>>     size <integer_cst 0x7ffff18a5e70 type <integer_type 0x7ffff18a9150
>> bitsizetype> constant 32>
>>     unit size <integer_cst 0x7ffff18a5e88 type <integer_type
>> 0x7ffff18a90a8 sizetype> constant 4>
>>     align 32 symtab 0 alias set 1 canonical type 0x7ffff18a9690
>> precision 32 min <integer_cst 0x7ffff18c60c0 -2147483648> max
>> <integer_cst 0x7ffff18c60d8 2147483647>
>>     pointer_to_this <pointer_type 0x7ffff18cb930>>
>> (gdb)
>>
>> This mismatch makes combine thinks a byte parameter is passed as int
>> in register and turns
>>
>> (insn 9 6 10 2 (set (reg:SI 92 [ b ])
>>         (zero_extend:SI (subreg:QI (reg:SI 91 [ b ]) 0))) pr64037.ii:9
>> 138 {*zero_extendqisi2}
>>      (expr_list:REG_DEAD (reg:SI 91 [ b ])
>>         (nil)))
>> (insn 10 9 0 2 (set (mem:SI (reg/v/f:SI 88 [ out ]) [1 *out_4(D)+0 S4
>> A32])
>>         (reg:SI 92 [ b ])) pr64037.ii:9 90 {*movsi_internal}
>>      (expr_list:REG_DEAD (reg:SI 92 [ b ])
>>         (expr_list:REG_DEAD (reg/v/f:SI 88 [ out ])
>>             (nil))))
>>
>> into
>>
>> Trying 9 -> 10:
>> Successfully matched this instruction:
>> (set (mem:SI (reg/v/f:SI 88 [ out ]) [1 *out_4(D)+0 S4 A32])
>>     (reg:SI 91 [ b ]))
>> allowing combination of insns 9 and 10
>> original costs 6 + 4 = 10
>> replacement cost 4
>> deferring deletion of insn with uid = 9.
>> modifying insn i3    10: [r88:SI]=r91:SI
>>       REG_DEAD r91:SI
>>       REG_DEAD r88:SI
>>
>>
>> This patch makes setup_incoming_promotions to match assign_parm_setup_reg.
>> Tested on Linux/x86-64 without regressions.  OK for trunk and backport?
>>
>> Thanks.
>>
>>
>> H.J.
>> ----
>> diff --git a/gcc/combine.c b/gcc/combine.c
>> index 1808f97..a0449a2 100644
>> --- a/gcc/combine.c
>> +++ b/gcc/combine.c
>> @@ -1561,8 +1561,8 @@ setup_incoming_promotions (rtx_insn *first)
>>        uns3 = TYPE_UNSIGNED (DECL_ARG_TYPE (arg));
>>
>>        /* The mode and signedness of the argument as it is actually passed,
>> -         after any TARGET_PROMOTE_FUNCTION_ARGS-driven ABI promotions.  */
>> -      mode3 = promote_function_mode (DECL_ARG_TYPE (arg), mode2, &uns3,
>> +         see assign_parm_setup_reg in function.c.  */
>> +      mode3 = promote_function_mode (TREE_TYPE (arg), mode1, &uns1,
>>                                      TREE_TYPE (cfun->decl), 0);
>>
>>        /* The mode of the register in which the argument is being passed.  */
>> diff --git a/gcc/testsuite/g++.dg/pr64037.C b/gcc/testsuite/g++.dg/pr64037.C
>> new file mode 100644
>> index 0000000..e5cd0e2
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/pr64037.C
>> @@ -0,0 +1,27 @@
>> +// { dg-do run { target i?86-*-* x86_64-*-* } }
>> +// { dg-options "-std=c++11 -Os" }
>> +
>> +enum class X : unsigned char {
>> +  V = 2,
>> +};
>> +
>> +static void
>> +__attribute__((noinline,noclone))
>> +foo(unsigned &out, unsigned a, X b)
>> +{
>> +  out = static_cast<unsigned>(b);
>> +}
>> +
>> +int main()
>> +{
>> +  unsigned deadbeef = 0xDEADBEEF;
>> +  asm volatile ("" : "+d" (deadbeef), "+c" (deadbeef));
>> +
>> +  unsigned out;
>> +  foo(out, 2, X::V);
>> +
>> +  if (out != 2)
>> +    __builtin_abort ();
>> +
>> +  return 0;
>> +}