First tiny step to implementing PR9322: build infrastructure for only emitting the
live case of a switch statement when switching on a constant. This is terribly
limited, but enough to handle the trivial example included. Before we would
emit:
define void @test1(i32 %i) nounwind {
entry:
%i.addr = alloca i32, align 4
store i32 %i, i32* %i.addr, align 4
switch i32 1, label %sw.epilog [
i32 1, label %sw.bb
]
sw.bb: ; preds = %entry
%tmp = load i32* %i.addr, align 4
%inc = add nsw i32 %tmp, 1
store i32 %inc, i32* %i.addr, align 4
br label %sw.epilog
sw.epilog: ; preds = %sw.bb, %entry
switch i32 0, label %sw.epilog3 [
i32 1, label %sw.bb1
]
sw.bb1: ; preds = %sw.epilog
%tmp2 = load i32* %i.addr, align 4
%add = add nsw i32 %tmp2, 2
store i32 %add, i32* %i.addr, align 4
br label %sw.epilog3
sw.epilog3: ; preds = %sw.bb1, %sw.epilog
ret void
}
now we emit:
define void @test1(i32 %i) nounwind {
entry:
%i.addr = alloca i32, align 4
store i32 %i, i32* %i.addr, align 4
%tmp = load i32* %i.addr, align 4
%inc = add nsw i32 %tmp, 1
store i32 %inc, i32* %i.addr, align 4
ret void
}
This improves -O0 compile time (less IR to generate and shove through the code
generator) and the clever linux kernel people found a way to fail to build if we
don't do this optimization. This step isn't enough to handle the kernel case
though.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126597 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/switch-dce.c b/test/CodeGen/switch-dce.c
new file mode 100644
index 0000000..f06a3ed
--- /dev/null
+++ b/test/CodeGen/switch-dce.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -triple i386-unknown-unknown -O0 %s -emit-llvm -o - | FileCheck %s
+
+// CHECK: @test1
+// CHECK-NOT: switch
+// CHECK: add nsw i32 {{.*}}, 1
+// CHECK-NOT: switch
+// CHECK-NOT: add nsw i32
+// CHECK: ret void
+void test1() {
+ int i;
+ switch (1)
+ case 1:
+ ++i;
+
+ switch (0)
+ case 1:
+ i+=2;
+}