On 05/15/2016 06:45 PM, Kugan Vivekanandarajah wrote:
Hi Richard,
Now that stage1 is open, I would like to get the type promotion passes
reviewed again. I have tested the patches on aarch64, x86-64, and
ppc64le without any new execution failures. There some test-cases that
fails for patterns. I will address them after getting feedback on the
basic structure.
I find myself wondering if this will eliminate some of the cases where
Kai's type casting motion was useful. And just to be clear, that would
be a good thing.
1. When we promote SSA as part of promote_ssa, we either promote the
definition. Or create a copy stmt that is inserted after the stmt that
define it. i.e, we want to promote the SSA and reflect the promotion
on all the uses (we promote in place). We do this because, we don’t
want to change all the uses.
+/* Promote definition DEF to promoted type. If the stmt that defines def
+ is def_stmt, make the type of def promoted type. If the stmt is such
+ that, result of the def_stmt cannot be of promoted type, create a new_def
+ of the original_type and make the def_stmt assign its value to newdef.
+ Then, create a NOP_EXPR to convert new_def to def of promoted type.
+
+ For example, for stmt with original_type char and promoted_type int:
+ char _1 = mem;
+ becomes:
+ char _2 = mem;
+ int _1 = (int)_2;
When does this case happen, and how is this any better than PRE or other
elimination/code motion algorithms in improving the generated code?
I would hazard a guess that it could happen if you still needed the char
sized used in a small number of cases, but generally wanted to promote
most uses to int?
+
However, if the defining stmt has to be the last stmt in the basic
block (eg, stmt that can throw), and if there is more than one normal
edges where we use this value, we cant insert the copy in all the
edges. Please note that the copy stmt copes the value to promoted SSA
with the same name.
Therefore I had to return false in this case for promote_ssa and fixup
uses. I ran into this while testing ppc64le. I am sure it can happen
in other cases.
Right.
Jeff
