BasicAA: Recognize cyclic NoAlias phis

Enhances basic alias analysis to recognize phis whose first incoming values are
NoAlias and whose other incoming values are just the phi node itself through
some amount of recursion.

Example: With this change basicaa reports that ptr_phi and ptr_phi2 do not alias
each other.

bb:
 ptr = ptr2 + 1

loop:
  ptr_phi = phi [bb, ptr], [loop, ptr_plus_one]
  ptr2_phi = phi [bb, ptr2], [loop, ptr2_plus_one]
  ...
  ptr_plus_one = gep ptr_phi, 1
  ptr2_plus_one = gep ptr2_phi, 1

This enables the elimination of one load in code like the following:

extern int foo;

int test_noalias(int *ptr, int num, int* coeff) {
  int *ptr2 = ptr;
  int result = (*ptr++) * (*coeff--);
  while (num--) {
    *ptr2++ = *ptr;
    result +=  (*coeff--) * (*ptr++);
  }
  *ptr = foo;
  return result;
}

Part 2/2 of fix for PR13564.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163319 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 33 insertions, 0 deletions

diff --git a/test/Analysis/BasicAA/phi-speculation.ll b/test/Analysis/BasicAA/phi-speculation.ll
new file mode 100644
index 0000000..21c6592
--- /dev/null
+++ b/test/Analysis/BasicAA/phi-speculation.ll
@@ -0,0 +1,33 @@
+target datalayout =
+"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+
+; ptr_phi and ptr2_phi do not alias.
+; CHECK: NoAlias: i32* %ptr2_phi, i32* %ptr_phi
+
+define i32 @test_noalias(i32* %ptr2, i32 %count, i32* %coeff) {
+entry:
+  %ptr = getelementptr inbounds i32* %ptr2, i64 1
+  br label %while.body
+
+while.body:
+  %num = phi i32 [ %count, %entry ], [ %dec, %while.body ]
+  %ptr_phi = phi i32* [ %ptr, %entry ], [ %ptr_inc, %while.body ]
+  %ptr2_phi = phi i32* [ %ptr2, %entry ], [ %ptr2_inc, %while.body ]
+  %result.09 = phi i32 [ 0 , %entry ], [ %add, %while.body ]
+  %dec = add nsw i32 %num, -1
+  %0 = load i32* %ptr_phi, align 4
+  store i32 %0, i32* %ptr2_phi, align 4
+  %1 = load i32* %coeff, align 4
+  %2 = load i32* %ptr_phi, align 4
+  %mul = mul nsw i32 %1, %2
+  %add = add nsw i32 %mul, %result.09
+  %tobool = icmp eq i32 %dec, 0
+  %ptr_inc = getelementptr inbounds i32* %ptr_phi, i64 1
+  %ptr2_inc = getelementptr inbounds i32* %ptr2_phi, i64 1
+  br i1 %tobool, label %the_exit, label %while.body
+
+the_exit:
+  ret i32 %add
+}