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Improve comment for struct symbolic_number in bswap pass 

2016-11-25  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * tree-ssa-math-opts.c (struct symbolic_number): Improve comment.

From-SVN: r242870
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Thomas Preud'homme 2016-11-25 10:03:46 +00:00 committed by Thomas Preud'homme
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commit d8cc3d06f8
2 changed files with 26 additions and 15 deletions
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@ -1,3 +1,7 @@
2016-11-25 Thomas Preud'homme <thomas.preudhomme@arm.com>
* tree-ssa-math-opts.c (struct symbolic_number): Improve comment.
2016-11-25 Thomas Preud'homme <thomas.preudhomme@arm.com>
PR tree-optimization/77673

37
gcc/tree-ssa-math-opts.c
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@ -1931,25 +1931,32 @@ make_pass_cse_sincos (gcc::context *ctxt)
return new pass_cse_sincos (ctxt);
}
/* A symbolic number is used to detect byte permutation and selection
patterns. Therefore the field N contains an artificial number
consisting of octet sized markers:
/* A symbolic number structure is used to detect byte permutation and selection
patterns of a source. To achieve that, its field N contains an artificial
number consisting of BITS_PER_MARKER sized markers tracking where does each
byte come from in the source:
0 - target byte has the value 0
FF - target byte has an unknown value (eg. due to sign extension)
1..size - marker value is the target byte index minus one.
0 - target byte has the value 0
FF - target byte has an unknown value (eg. due to sign extension)
1..size - marker value is the byte index in the source (0 for lsb).
To detect permutations on memory sources (arrays and structures), a symbolic
number is also associated a base address (the array or structure the load is
made from), an offset from the base address and a range which gives the
difference between the highest and lowest accessed memory location to make
such a symbolic number. The range is thus different from size which reflects
the size of the type of current expression. Note that for non memory source,
range holds the same value as size.
number is also associated:
- a base address BASE_ADDR and an OFFSET giving the address of the source;
- a range which gives the difference between the highest and lowest accessed
memory location to make such a symbolic number;
- the address SRC of the source element of lowest address as a convenience
to easily get BASE_ADDR + offset + lowest bytepos.
For instance, for an array char a[], (short) a[0] | (short) a[3] would have
a size of 2 but a range of 4 while (short) a[0] | ((short) a[0] << 1) would
still have a size of 2 but this time a range of 1. */
Note 1: the range is different from size as size reflects the size of the
type of the current expression. For instance, for an array char a[],
(short) a[0] | (short) a[3] would have a size of 2 but a range of 4 while
(short) a[0] | ((short) a[0] << 1) would still have a size of 2 but this
time a range of 1.
Note 2: for non-memory sources, range holds the same value as size.
Note 3: SRC points to the SSA_NAME in case of non-memory source. */
struct symbolic_number {
uint64_t n;