Analyze recursive PHI nodes in BasicAA

Summary:
This patch allows phi nodes like
  %x = phi [ %incptr, ... ] [ %var, ... ]
  %incptr = getelementptr %x, 1
to be analyzed by BasicAliasAnalysis.

In aliasPHI, we can detect incoming values that are recursive GEPs with a
constant offset. Instead of trying to analyze a recursive GEP (and failing), 
we now ignore it and instead set the size of the memory referenced by
the PHINode to UnknownSize. This represents all the possible memory
locations the pointer represented by the PHINode could be advanced to
by the GEP.

For now, this new behavior is turned off by default to allow debugging of
performance degradations seen with SPEC/x86 and Hexagon benchmarks.
The flag -basicaa-recphi turns it on.


Reviewers: hfinkel, sanjoy

Subscribers: tobiasvk_caf, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D10368

llvm-svn: 242320

1 files changed, 75 insertions, 0 deletions

diff --git a/llvm/test/Analysis/BasicAA/phi-loop.ll b/llvm/test/Analysis/BasicAA/phi-loop.ll
new file mode 100644
index 00000000000..6337bfbc1cf
--- /dev/null
+++ b/llvm/test/Analysis/BasicAA/phi-loop.ll
@@ -0,0 +1,75 @@
+; RUN: opt < %s -basicaa -basicaa-recphi=1 -gvn -S | FileCheck %s
+;
+; Check that section->word_ofs doesn't get reloaded in every iteration of the
+; for loop.
+;
+; Code:
+;
+; typedef struct {
+;   unsigned num_words;
+;   unsigned word_ofs;
+;   const unsigned *data;
+; } section_t;
+; 
+; 
+; void test2(const section_t * restrict section, unsigned * restrict dst) {;
+;   while (section->data != NULL) {
+;     const unsigned *src = section->data;
+;     for (unsigned i=0; i < section->num_words; ++i) {
+;       dst[section->word_ofs + i] = src[i];
+;     }   
+; 
+;     ++section;
+;   }
+; }
+; 
+
+; CHECK-LABEL: for.body:
+; CHECK-NOT: load i32, i32* %word_ofs
+
+%struct.section_t = type { i32, i32, i32* }
+
+define void @test2(%struct.section_t* noalias nocapture readonly %section, i32* noalias nocapture %dst) {
+entry:
+  %data13 = getelementptr inbounds %struct.section_t, %struct.section_t* %section, i32 0, i32 2
+  %0 = load i32*, i32** %data13, align 4
+  %cmp14 = icmp eq i32* %0, null
+  br i1 %cmp14, label %while.end, label %for.cond.preheader
+
+for.cond.preheader:                               ; preds = %entry, %for.end
+  %1 = phi i32* [ %6, %for.end ], [ %0, %entry ]
+  %section.addr.015 = phi %struct.section_t* [ %incdec.ptr, %for.end ], [ %section, %entry ]
+  %num_words = getelementptr inbounds %struct.section_t, %struct.section_t* %section.addr.015, i32 0, i32 0
+  %2 = load i32, i32* %num_words, align 4
+  %cmp211 = icmp eq i32 %2, 0
+  br i1 %cmp211, label %for.end, label %for.body.lr.ph
+
+for.body.lr.ph:                                   ; preds = %for.cond.preheader
+  %word_ofs = getelementptr inbounds %struct.section_t, %struct.section_t* %section.addr.015, i32 0, i32 1
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.lr.ph, %for.body
+  %arrayidx.phi = phi i32* [ %1, %for.body.lr.ph ], [ %arrayidx.inc, %for.body ]
+  %i.012 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
+  %3 = load i32, i32* %arrayidx.phi, align 4
+  %4 = load i32, i32* %word_ofs, align 4
+  %add = add i32 %4, %i.012
+  %arrayidx3 = getelementptr inbounds i32, i32* %dst, i32 %add
+  store i32 %3, i32* %arrayidx3, align 4
+  %inc = add i32 %i.012, 1
+  %5 = load i32, i32* %num_words, align 4
+  %cmp2 = icmp ult i32 %inc, %5
+  %arrayidx.inc = getelementptr i32, i32* %arrayidx.phi, i32 1
+  br i1 %cmp2, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body, %for.cond.preheader
+  %incdec.ptr = getelementptr inbounds %struct.section_t, %struct.section_t* %section.addr.015, i32 1
+  %data = getelementptr inbounds %struct.section_t, %struct.section_t* %section.addr.015, i32 1, i32 2
+  %6 = load i32*, i32** %data, align 4
+  %cmp = icmp eq i32* %6, null
+  br i1 %cmp, label %while.end, label %for.cond.preheader
+
+while.end:                                        ; preds = %for.end, %entry
+  ret void
+}
+