On Thu, Oct 3, 2024 at 7:51 AM Filip Kastl <fka...@suse.cz> wrote:
>
> Hi All,
>
> While toying with the switch conversion GIMPLE pass I noticed that the pass
> generates a dead statement. I wanted to investigate why this happens and
> potentially fix this. However after looking into the part of the pass
> responsible for generating the code in question I still have no idea why the
> dead statement is generated. Can someone more experienced look at this and
> tell me what is going on, please?
>
> Let's say I'm compiling this C testcase
>
> int main()
> {
> switch (a % 4)
> {
> case 0: return 10;
> case 1: return 20;
> case 2: return 30;
> default: return 40;
> }
> }
>
> Since the switch computes a linear function, the linear transformation feature
> of switch conversion triggers -- with cofficients A = 10 and B = 10 in this
> case. This is the relevant GCC source code. It is a part of the
> switch_conversion::build_one_array () function:
>
> if (dump_file && coeff_a.to_uhwi () > 0)
> fprintf (dump_file, "Linear transformation with A = %" PRId64
> " and B = %" PRId64 "\n", coeff_a.to_shwi (),
> coeff_b.to_shwi ());
>
> /* We must use type of constructor values. */
> gimple_seq seq = NULL;
> tree tmp = gimple_convert (&seq, type, m_index_expr);
> tree tmp2 = gimple_build (&seq, MULT_EXPR, type,
> wide_int_to_tree (type, coeff_a), tmp);
> tree tmp3 = gimple_build (&seq, PLUS_EXPR, type, tmp2,
> wide_int_to_tree (type, coeff_b));
> tree tmp4 = gimple_convert (&seq, TREE_TYPE (name), tmp3);
> gsi_insert_seq_before (&gsi, seq, GSI_SAME_STMT);
> load = gimple_build_assign (name, tmp4);
Both gimple_build uses gimple_simplify to build the gimple (which does
simplifications).
Note gimple_convert just calls gimple_build (unless it is already the
correct type).
You can find which simplifications is done by adding the `-folding`
option to the dump option (note this option is not documented but that
is PR 114892).
Thanks,
Andrew Pinski
>
> Before this code is run, the GIMPLE of the basic block in question looks like
> this (output of debug_bb (m_switch_bb)):
>
> a.0_1 = a;
> _2 = a.0_1 % 4;
> _9 = (unsigned int) _2;
> switch (_2) <default: <L4> [INV], case 0: <L6> [INV], case 1: <L1> [INV],
> case 2: <L2> [INV], case 3: <L3> [INV]>
>
> What I would expect to see is something like this (also output of debug_bb ()
> but this time after the code I listed was run):
>
> a.0_1 = a;
> _2 = a.0_1 % 4;
> _9 = (unsigned int) _2;
> _7 = (unsigned int) _2;
> _8 = 10 * _7;
> _10 = _8 + 10;
> switch (_2) <default: <L4> [INV], case 0: <L6> [INV], case 1: <L1> [INV],
> case 2: <L2> [INV], case 3: <L3> [INV]>
>
> but what I instead see is this:
>
> a.0_1 = a;
> _2 = a.0_1 % 4;
> _9 = (unsigned int) _2;
> _7 = (unsigned int) _2;
> _6 = 10 * _7;
> _5 = _7 + 1;
> _10 = _5 * 10;
> _11 = (int) _10;
> switch (_2) <default: <L4> [INV], case 0: <L6> [INV], case 1: <L1> [INV],
> case 2: <L2> [INV], case 3: <L3> [INV]>
>
> The first thing I noticed is that there are two multiplications instead of one
> and that the result of one of them doesn't get used (this redundant
> multiplication is the original reason I started looking into this). But there
> is also a cast to int that I don't see how the GCC code created and the order
> of the non-dead multiplication and addition is switched and the added constant
> (coefficient B) is 1 instead of 10 (which is correct but just not what I
> expect
> from reading the code).
>
> What am I not seeing here? Does gsi_insert_seq_before do some optimizations
> on
> the seq it inserts?
>
> Thanks,
> Filip Kastl
