[PATCH] Fix a performance problem in gep(gep ...) merging

[Wei Mi]

Hi nicholas, qcolombet,

The patch is to fix the performance problem described in https://llvm.org/bugs/show_bug.cgi?id=23163.

As described in the bug above, gep merging may have negative effect on performance, especially when the src of gep has multiple uses, so we want to make sure every target gep after the merging has lower cost than before.  In the patch, we achieve it by requiring that the index values of the src and target geps to be merged are both constants.

The patch is performance neutral for spec2000 on x86 sandybridge. It improves google internal benchmarks by 0.6 on x86 sandybridge and 0.4 on x86 westmere in geomean.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D8911

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/gep-merge.ll

Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1469,0 +1470,5 @@
+        // Only do the combine when GO1 and SO1 are both constants. In this
+        // case, we are sure the cost after the merge is never higher than
+        // before.
+        if (!isa<Constant>(GO1) || !isa<Constant>(SO1))
+          return nullptr;
Index: test/Transforms/InstCombine/gep-merge.ll
===================================================================
--- test/Transforms/InstCombine/gep-merge.ll
+++ test/Transforms/InstCombine/gep-merge.ll
@@ -0,0 +1,48 @@
+; This test makes sure that gep(gep ...) merge doesn't come into effect.
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; Make sure there are no geps being merged.
+; CHECK-LABEL: @fn3(
+; CHECK: getelementptr
+; CHECK: getelementptr
+; CHECK: getelementptr
+
+ at _ZN2cv1aE = global i8* zeroinitializer, align 8
+declare i32 @fn1() #2
+declare i32 @fn2() #2
+
+; Function Attrs: uwtable
+define linkonce_odr i32 @fn3() {
+entry:
+  %call = call i32 @fn1()
+  %call1 = call i32 @fn2()
+  %0 = load i8*, i8** @_ZN2cv1aE, align 8
+  %idx.ext2 = sext i32 %call1 to i64
+  %add.ptr3 = getelementptr inbounds i8, i8* %0, i64 %idx.ext2
+  br label %for.cond5
+
+for.cond5:
+  %total1 = phi i32 [ 0, %entry ], [ %total2, %for.body7 ]
+  %x.1 = phi i32 [ 0, %entry ], [ %inc, %for.body7 ]
+  %cmp6 = icmp slt i32 %x.1, %call
+  br i1 %cmp6, label %for.body7, label %for.cond34
+
+for.body7:                                        ; preds = %for.cond5
+  %mul = mul nsw i32 %x.1, 2
+  %idxprom = sext i32 %mul to i64
+  %arrayidx = getelementptr inbounds i8, i8* %add.ptr3, i64 %idxprom
+  %1 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %1 to i32
+  %sub = sub nsw i32 %mul, 1
+  %idxprom10 = sext i32 %sub to i64
+  %arrayidx11 = getelementptr inbounds i8, i8* %add.ptr3, i64 %idxprom10
+  %2 = load i8, i8* %arrayidx11, align 1
+  %conv2 = zext i8 %2 to i32
+  %add1 = add nsw i32 %conv, %conv2
+  %total2 = add nsw i32 %total1, %add1
+  %inc = add nsw i32 %x.1, 1
+  br label %for.cond5
+
+for.cond34:
+  ret i32 %total1
+}

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