From 68081f41fafc13e194dfa639a3219b21c176489f Mon Sep 17 00:00:00 2001
From: Sanjoy Das <sanjoy@playingwithpointers.com>
Date: Sat, 6 Jun 2015 05:24:10 +0000
Subject: [PATCH] [LoopUnroll] Fix truncation bug in canUnrollCompletely.

Summary:
canUnrollCompletely takes `unsigned` values for `UnrolledCost` and
`RolledDynamicCost` but is passed in `uint64_t`s that are silently
truncated.  Because of this, when `UnrolledSize` is a large integer
that has a small remainder with UINT32_MAX, LLVM tries to completely
unroll loops with high trip counts.

Reviewers: mzolotukhin, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10293

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239218 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/LoopUnrollPass.cpp      |  6 +-
 .../LoopUnroll/full-unroll-bad-cost.ll        | 58 +++++++++++++++++++
 2 files changed, 61 insertions(+), 3 deletions(-)
 create mode 100644 test/Transforms/LoopUnroll/full-unroll-bad-cost.ll

diff --git a/lib/Transforms/Scalar/LoopUnrollPass.cpp b/lib/Transforms/Scalar/LoopUnrollPass.cpp
index d8ba9648b95..e831d834070 100644
--- a/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -227,7 +227,7 @@ namespace {
     bool canUnrollCompletely(Loop *L, unsigned Threshold,
                              unsigned PercentDynamicCostSavedThreshold,
                              unsigned DynamicCostSavingsDiscount,
-                             unsigned UnrolledCost, unsigned RolledDynamicCost);
+                             uint64_t UnrolledCost, uint64_t RolledDynamicCost);
   };
 }
 
@@ -768,8 +768,8 @@ static void SetLoopAlreadyUnrolled(Loop *L) {
 bool LoopUnroll::canUnrollCompletely(Loop *L, unsigned Threshold,
                                      unsigned PercentDynamicCostSavedThreshold,
                                      unsigned DynamicCostSavingsDiscount,
-                                     unsigned UnrolledCost,
-                                     unsigned RolledDynamicCost) {
+                                     uint64_t UnrolledCost,
+                                     uint64_t RolledDynamicCost) {
 
   if (Threshold == NoThreshold) {
     DEBUG(dbgs() << "  Can fully unroll, because no threshold is set.\n");
diff --git a/test/Transforms/LoopUnroll/full-unroll-bad-cost.ll b/test/Transforms/LoopUnroll/full-unroll-bad-cost.ll
new file mode 100644
index 00000000000..e5694fbeb0c
--- /dev/null
+++ b/test/Transforms/LoopUnroll/full-unroll-bad-cost.ll
@@ -0,0 +1,58 @@
+; RUN: opt -S -loop-unroll < %s | FileCheck %s
+
+; LLVM should not try to fully unroll this loop.
+
+declare void @f()
+declare void @g()
+declare void @h()
+
+define void @trivial_loop() {
+; CHECK-LABEL: @trivial_loop(
+ entry:
+  br label %loop
+
+ loop:
+  %idx = phi i32 [ 0, %entry ], [ %idx.inc, %loop ]
+  %idx.inc = add i32 %idx, 1
+  call void @f()
+  call void @g()
+  call void @h()
+  call void @f()
+  call void @g()
+  call void @h()
+  call void @f()
+  call void @g()
+  call void @h()
+  call void @f()
+  call void @g()
+  call void @h()
+  call void @f()
+  call void @g()
+  call void @h()
+  %be = icmp slt i32 %idx, 268435456
+  br i1 %be, label %loop, label %exit
+
+; CHECK: loop:
+; CHECK-NEXT:  %idx = phi i32 [ 0, %entry ], [ %idx.inc, %loop ]
+; CHECK-NEXT:  %idx.inc = add i32 %idx, 1
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @h()
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @h()
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @h()
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @h()
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  call void @h()
+; CHECK-NEXT:  %be = icmp slt i32 %idx, 268435456
+; CHECK-NEXT:  br i1 %be, label %loop, label %exit
+
+ exit:
+  ret void
+}
-- 
2.34.1