[MDA] change BlockScanLimit into a command line option.

[wengxt]

Summary

In the benchmark (https://github.com/vetter/shoc) we are researching,
the duplicated load is not eliminated because MemoryDependenceAnalysis
hit the BlockScanLimit. This patch change it into a command line option
instead of a hardcoded value.

Patched by Xuetian Weng.

Test Plan: test/Analysis/MemoryDependenceAnalysis/memdep-block-scan-limit.ll

[reames]

LGTM

lib/Analysis/MemoryDependenceAnalysis.cpp

 STATISTIC(NumCacheDirtyNonLocalPtr,
           "Number of cached, but dirty, non-local ptr responses");
 STATISTIC(NumUncacheNonLocalPtr,
           "Number of uncached non-local ptr responses");
 STATISTIC(NumCacheCompleteNonLocalPtr,
           "Number of block queries that were completely cached");
 
 // Limit for the number of instructions to scan in a block.
-static const unsigned int BlockScanLimit = 100;
+
+static cl::opt<unsigned> BlockScanLimit(
+    "memdep-block-scan-limit", cl::Hidden, cl::init(100),
+    cl::desc("The number of instructions to scan in a block in memory "
+             "dependency analysis (default = 100)"));
 
 // Limit on the number of memdep results to process.
 static const unsigned int NumResultsLimit = 100;
 
 char MemoryDependenceAnalysis::ID = 0;
 
 // Register this pass...
 INITIALIZE_PASS_BEGIN(MemoryDependenceAnalysis, "memdep",

test/Analysis/MemoryDependenceAnalysis/memdep-block-scan-limit.ll

+; RUN: opt -S -memdep -gvn -basicaa < %s | FileCheck %s
+; RUN: opt -S -memdep -memdep-block-scan-limit=1 -gvn -basicaa < %s | FileCheck %s --check-prefix=WITH-LIMIT
+; CHECK-LABEL: @test(
+; CHECK: load
+; CHECK-NOT: load
+; WITH-LIMIT-LABEL: @test(
+; WITH-LIMIT-CHECK: load
+; WITH-LIMIT-CHECK: load
+define i32 @test(i32* %p) {
+ %1 = load i32, i32* %p
+ %2 = add i32 %1, 3
+ %3 = load i32, i32* %p
+ %4 = add i32 %2, %3
+ ret i32 %4
+}