@@ -1925,25 +1925,32 @@ 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.
-
- 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. */
+ 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.
+
+ 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;
Hi, Fix in r242869 for PR77673 was accompanied with r242870 which updated the description of the struct symbolic_number modified by the previous patch. It did so by rewriting the comment completely but I believe this patch should be still backported to make the comment match the code. ChangeLog entry is as follows: *** gcc/ChangeLog *** 2016-12-12 Thomas Preud'homme <thomas.preudhomme@arm.com> Backport from mainline 2016-11-25 Thomas Preud'homme <thomas.preudhomme@arm.com> * tree-ssa-math-opts.c (struct symbolic_number): Improve comment. Is this ok for gcc-5-branch and gcc-6-branch? Best regards, Thomas