From 056922ee4181f19d441415d01ffd99604cd97da5 Mon Sep 17 00:00:00 2001
From: Kirill Naumov <knaumov@azul.com>
Date: Tue, 2 Jun 2020 19:22:41 +0000
Subject: [PATCH] [InlineCost] GetElementPtr with constant operands

If the GEP instruction contanins only constants as its arguments,
then it should be recognized as a constant. For now, there was
also added a flag to turn off this simplification if it causes
any regressions ("disable-gep-const-evaluation") which is off
by default. Once I gather needed data of the effectiveness of
this simplification, the flag will be deleted.

Reviewers: apilipenko, davidxl, mtrofin

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D81026
---
 lib/Analysis/InlineCost.cpp                 | 14 ++++++++++++++
 test/Transforms/Inline/gep_from_constant.ll | 15 +++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 test/Transforms/Inline/gep_from_constant.ll

diff --git a/lib/Analysis/InlineCost.cpp b/lib/Analysis/InlineCost.cpp
index c05d1ee7e9a..33d714406d7 100644
--- a/lib/Analysis/InlineCost.cpp
+++ b/lib/Analysis/InlineCost.cpp
@@ -110,6 +110,10 @@ static cl::opt<bool> InlineCallerSupersetNoBuiltin(
     cl::desc("Allow inlining when caller has a superset of callee's nobuiltin "
              "attributes."));
 
+static cl::opt<bool> DisableGEPConstOperand(
+    "disable-gep-const-evaluation", cl::Hidden, cl::init(false),
+    cl::desc("Disables evaluation of GetElementPtr with constant operands"));
+
 namespace {
 class InlineCostCallAnalyzer;
 
@@ -1019,6 +1023,16 @@ bool CallAnalyzer::visitGetElementPtr(GetElementPtrInst &I) {
     return true;
   };
 
+  if (!DisableGEPConstOperand)
+    if (simplifyInstruction(I, [&](SmallVectorImpl<Constant *> &COps) {
+        SmallVector<Constant *, 2> Indices;
+        for (unsigned int Index = 1 ; Index < COps.size() ; ++Index)
+            Indices.push_back(COps[Index]);
+        return ConstantExpr::getGetElementPtr(I.getSourceElementType(), COps[0],
+                                              Indices, I.isInBounds());
+        }))
+      return true;
+
   if ((I.isInBounds() && canFoldInboundsGEP(I)) || IsGEPOffsetConstant(I)) {
     if (SROAArg)
       SROAArgValues[&I] = SROAArg;
diff --git a/test/Transforms/Inline/gep_from_constant.ll b/test/Transforms/Inline/gep_from_constant.ll
new file mode 100644
index 00000000000..eafb4710690
--- /dev/null
+++ b/test/Transforms/Inline/gep_from_constant.ll
@@ -0,0 +1,15 @@
+; RUN: opt < %s -passes="print<inline-cost>" 2>&1 | FileCheck %s
+
+; CHECK-LABEL: @foo
+; CHECK: cost before = {{.*}}, cost after = {{.*}}, threshold before = {{.*}}, threshold after = {{.*}}, cost delta = {{.*}}, simplified to i8 addrspace(1)** getelementptr (i8 addrspace(1)*, i8 addrspace(1)** inttoptr (i64 754974720 to i8 addrspace(1)**), i64 5)
+
+define i8 addrspace(1)** @foo(i64 %0) {
+  %2 = inttoptr i64 754974720 to i8 addrspace(1)**
+  %3 = getelementptr i8 addrspace(1)*, i8 addrspace(1)** %2, i64 %0
+  ret i8 addrspace(1)** %3
+}
+
+define i8 addrspace(1)** @main() {
+  %1 = call i8 addrspace(1)** @foo(i64 5)
+  ret i8 addrspace(1)** %1
+}