[llvm] r362711 - [DA] Add an option to control delinearization validity checks

[Whitney Tsang via llvm-commits]

Author: whitneyt
Date: Thu Jun  6 08:12:49 2019
New Revision: 362711

URL: http://llvm.org/viewvc/llvm-project?rev=362711&view=rev
Log:
[DA] Add an option to control delinearization validity checks

Summary: Dependence Analysis performs static checks to confirm validity
of delinearization. These checks often fail for 64-bit targets due to
type conversions and integer wrapping that prevent simplification of the
SCEV expressions. These checks would also fail at compile-time if the
lower bound of the loops are compile-time unknown.

For example:

void foo(int n, int m, int a[][m]) {
  for (int i = 0; i < n; ++i)
    for (int j = 0; j < m; ++j) {
      a[i][j] = a[i+1][j-2];
    }
}

opt -mem2reg -instcombine -indvars -loop-simplify -loop-rotate -inline
-pass-remarks=.* -debug-pass=Arguments
-da-permissive-validity-checks=false k3.ll -analyze -da
will produce the following by default:

da analyze - anti [* *|<]!
but will produce the following expected dependence vector if the
validity checks are disabled:

da analyze - consistent anti [1 -2]!
This revision will introduce a debug option that will leave the validity
checks in place by default, but allow them to be turned off. New tests
are added for cases where it cannot be proven at compile-time that the
individual subscripts stay in-bound with respect to a particular
dimension of an array. These tests enable the option to provide user
guarantee that the subscripts do not over/under-flow into other
dimensions, thereby producing more accurate dependence vectors.

For prior discussion on this topic, leading to this change, please see
the following thread:
http://lists.llvm.org/pipermail/llvm-dev/2019-May/132372.html

Reviewers: Meinersbur, jdoerfert, kbarton, dmgreen, fhahn
Reviewed By: Meinersbur, jdoerfert, dmgreen
Subscribers: fhahn, hiraditya, javed.absar, llvm-commits, Whitney,
etiotto
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D62610

Modified:
    llvm/trunk/lib/Analysis/DependenceAnalysis.cpp

Modified: llvm/trunk/lib/Analysis/DependenceAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/DependenceAnalysis.cpp?rev=362711&r1=362710&r2=362711&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/DependenceAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/DependenceAnalysis.cpp Thu Jun  6 08:12:49 2019
@@ -109,6 +109,14 @@ STATISTIC(BanerjeeSuccesses, "Banerjee s
 static cl::opt<bool>
     Delinearize("da-delinearize", cl::init(true), cl::Hidden, cl::ZeroOrMore,
                 cl::desc("Try to delinearize array references."));
+static cl::opt<bool> DisableDelinearizationChecks(
+    "da-disable-delinearization-checks", cl::init(false), cl::Hidden,
+    cl::ZeroOrMore,
+    cl::desc(
+        "Disable checks that try to statically verify validity of "
+        "delinearized subscripts. Enabling this option may result in incorrect "
+        "dependence vectors for languages that allow the subscript of one "
+        "dimension to underflow or overflow into another dimension."));
 
 //===----------------------------------------------------------------------===//
 // basics
@@ -3316,19 +3324,20 @@ bool DependenceInfo::tryDelinearize(Inst
   // and dst.
   // FIXME: It may be better to record these sizes and add them as constraints
   // to the dependency checks.
-  for (int i = 1; i < size; ++i) {
-    if (!isKnownNonNegative(SrcSubscripts[i], SrcPtr))
-      return false;
+  if (!DisableDelinearizationChecks)
+    for (int i = 1; i < size; ++i) {
+      if (!isKnownNonNegative(SrcSubscripts[i], SrcPtr))
+        return false;
 
-    if (!isKnownLessThan(SrcSubscripts[i], Sizes[i - 1]))
-      return false;
+      if (!isKnownLessThan(SrcSubscripts[i], Sizes[i - 1]))
+        return false;
 
-    if (!isKnownNonNegative(DstSubscripts[i], DstPtr))
-      return false;
+      if (!isKnownNonNegative(DstSubscripts[i], DstPtr))
+        return false;
 
-    if (!isKnownLessThan(DstSubscripts[i], Sizes[i - 1]))
-      return false;
-  }
+      if (!isKnownLessThan(DstSubscripts[i], Sizes[i - 1]))
+        return false;
+    }
 
   LLVM_DEBUG({
     dbgs() << "\nSrcSubscripts: ";