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some more PRE/GVN/DSE related notes.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60633 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2008-12-06 22:52:12 +00:00
parent f96ca79cf3
commit 6a09a74e46
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lib/Target/README.txt
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@ -1307,6 +1307,9 @@ bb2: ; preds = %bb, %bb1
DSE should sink partially dead stores to get the store out of the loop.
Here's another partial dead case:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12395
//===---------------------------------------------------------------------===//
Scalar PRE hoists the mul in the common block up to the else:
@ -1367,6 +1370,54 @@ bb3: ; preds = %bb1, %bb2, %bb
%11 is fully redundant, an in BB2 it should have the value %8.
GCC PR33344 is a similar case.
//===---------------------------------------------------------------------===//
There are many load PRE testcases in testsuite/gcc.dg/tree-ssa/loadpre* in the
GCC testsuite. There are many pre testcases as ssa-pre-*.c
Other simple load PRE cases:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35287
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34677 (licm does this)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29789 (SPEC2K6)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23455
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14705
//===---------------------------------------------------------------------===//
When GVN/PRE finds a store of float* to a must aliases pointer when expecting
an int*, it should turn it into a bitcast. This is a nice generalization of
the SROA hack that would apply to other cases.
One example (that requires crazy phi translation) is:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16799
//===---------------------------------------------------------------------===//
A/B get pinned to the stack because we turn an if/then into a select instead
of PRE'ing the load/store. This may be fixable in instcombine:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37892
Interesting missed case because of control flow flattening (should be 2 loads):
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26629
//===---------------------------------------------------------------------===//
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19633
We could eliminate the branch condition here, loading from null is undefined:
struct S { int w, x, y, z; };
struct T { int r; struct S s; };
void bar (struct S, int);
void foo (int a, struct T b)
{
struct S *c = 0;
if (a)
c = &b.s;
bar (*c, a);
}
//===---------------------------------------------------------------------===//