RPCS3/llvm-mirror
1
0
Fork 0
mirror of https://github.com/RPCS3/llvm-mirror.git synced 2025-01-31 12:41:49 +01:00
Code Issues Projects Releases Wiki Activity

[LAA] Add clarifying comments for the checking pointer grouping algorithm. NFC

Browse Source 
llvm-svn: 243416
This commit is contained in:
Silviu Baranga 2015-07-28 13:44:08 +00:00
parent b3b937fabe
commit 97f890ebca
2 changed files with 31 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
lib/Analysis/LoopAccessAnalysis.cpp
View File

@ -237,8 +237,31 @@ void RuntimePointerChecking::groupChecks(
CheckingGroups.clear();
// If we need to check two pointers to the same underlying object
// with a non-constant difference, we shouldn't perform any pointer
// grouping with those pointers. This is because we can easily get
// into cases where the resulting check would return false, even when
// the accesses are safe.
//
// The following example shows this:
// for (i = 0; i < 1000; ++i)
// a[5000 + i * m] = a[i] + a[i + 9000]
//
// Here grouping gives a check of (5000, 5000 + 1000 * m) against
// (0, 10000) which is always false. However, if m is 1, there is no
// dependence. Not grouping the checks for a[i] and a[i + 9000] allows
// us to perform an accurate check in this case.
//
// The above case requires that we have an UnknownDependence between
// accesses to the same underlying object. This cannot happen unless
// ShouldRetryWithRuntimeCheck is set, and therefore UseDependencies
// is also false. In this case we will use the fallback path and create
// separate checking groups for all pointers.
// If we don't have the dependency partitions, construct a new
// checking pointer group for each pointer.
// checking pointer group for each pointer. This is also required
// for correctness, because in this case we can have checking between
// pointers to the same underlying object.
if (!UseDependencies) {
for (unsigned I = 0; I < Pointers.size(); ++I)
CheckingGroups.push_back(CheckingPtrGroup(I, *this));

9
test/Analysis/LoopAccessAnalysis/number-of-memchecks.ll
View File

@ -217,8 +217,9 @@ for.end: ; preds = %for.body
ret void
}
; Don't merge pointers if there is some other check which could be falsely
; invalidated. For example, in the following loop:
; Don't merge pointers if we need to perform a check against a pointer
; to the same underlying object (doing so would emit a check that could be
; falsely invalidated) For example, in the following loop:
;
; for (i = 0; i < 5000; ++i)
; a[i + offset] = a[i] + a[i + 10000]
@ -226,6 +227,10 @@ for.end: ; preds = %for.body
; we should not merge the intervals associated with the reads (0,5000) and
; (10000, 15000) into (0, 15000) as this will pottentially fail the check
; against the interval associated with the write.
;
; We cannot have this check unless ShouldRetryWithRuntimeCheck is set,
; and therefore the grouping algorithm would create a separate group for
; each pointer.
; CHECK: function 'testi':
; CHECK: Run-time memory checks: