From 4a7da98bd9283e2e6aa635031d2f131493c8ad1a Mon Sep 17 00:00:00 2001
From: Christian Bruel <christian.bruel@st.com>
Date: Thu, 23 May 2019 05:53:10 +0000
Subject: [GlobalOpt] recognize dead struct fields and propagate values

Summary:
Allow struct fields SRA and dead stores. This works by considering fields accesses from getElementPtr to be considered as a possible pointer root that can be cleaned up.
We check that the variable can be SRA by recursively checking the sub expressions with the new isSafeSubSROAGEP function.

basically this allows the array in following C code  to be optimized out

struct Expr {
  int a[2];
  int b;
};

static struct Expr e;

int foo (int i)
{
  e.b = 2;
  e.a[i] = 1;
  return e.b;
}


Reviewers: greened, bkramer, nicholas, jmolloy

Reviewed By: jmolloy

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61911

llvm-svn: 361460
---
 llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll | 11 +++++++++--
 llvm/test/Transforms/GlobalOpt/globalsra-struct.ll   | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/Transforms/GlobalOpt/globalsra-struct.ll

(limited to 'llvm/test')

diff --git a/llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll b/llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll
index 87a8486d881..c32a620c476 100644
--- a/llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll
+++ b/llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll
@@ -4,13 +4,20 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @g_data = internal unnamed_addr global <{ [8 x i16], [8 x i16] }> <{ [8 x i16] [i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16], [8 x i16] zeroinitializer }>, align 16
-; We cannot SRA here due to the second gep meaning the access to g_data may be to either element
-; CHECK: @g_data = internal unnamed_addr constant <{ [8 x i16], [8 x i16] }>
+; We normally cannot SRA here due to the second gep meaning the access to g_data may be to either element,
+; unless the value is always zero.
+; CHECK: @g_data.0 = internal unnamed_addr constant [8 x i16] [i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16], align 16
 
 define i16 @test(i64 %a1) {
 entry:
   %g1 = getelementptr inbounds <{ [8 x i16], [8 x i16] }>, <{ [8 x i16], [8 x i16] }>* @g_data, i64 0, i32 0
   %arrayidx.i = getelementptr inbounds [8 x i16], [8 x i16]* %g1, i64 0, i64 %a1
   %r = load i16, i16* %arrayidx.i, align 2
+
+; CHECK-NOT: getelementptr inbounds <{ [8 x i16], [8 x i16] }>, <{ [8 x i16], [8 x i16] }>* @g_data, i64 0, i32 0
+; CHECK:  %arrayidx.i = getelementptr inbounds [8 x i16], [8 x i16]* @g_data.0, i64 0, i64 %a1
+
   ret i16 %r
+
+
 }
diff --git a/llvm/test/Transforms/GlobalOpt/globalsra-struct.ll b/llvm/test/Transforms/GlobalOpt/globalsra-struct.ll
new file mode 100644
index 00000000000..957fba81068
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/globalsra-struct.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -globalopt -S | FileCheck %s
+
+%struct.Expr = type { [1 x i32], i32 }
+
+@e = internal global %struct.Expr zeroinitializer, align 4
+; CHECK-NOT: @e = internal global %struct.Expr zeroinitializer, align 4
+
+define dso_local i32 @foo(i32 %i) {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  %0 = load i32, i32* %i.addr, align 4
+  %arrayidx = getelementptr inbounds [1 x i32], [1 x i32]* getelementptr inbounds (%struct.Expr, %struct.Expr* @e, i32 0, i32 0), i32 0, i32 %0
+  store i32 57005, i32* %arrayidx, align 4
+  %1 = load i32, i32* getelementptr inbounds (%struct.Expr, %struct.Expr* @e, i32 0, i32 1), align 4
+  ret i32 %1
+; CHECK:  ret i32 0
+}
-- 
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