[clang] [Clang] Add fake use emission to Clang with -fextend-lifetimes (PR #110102)

[Stephen Tozer via cfe-commits]

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@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 %s -O2 -emit-llvm -fextend-lifetimes -o - | FileCheck %s
+// Check that fake use calls are emitted at the correct locations, i.e.
+// at the end of lexical blocks and at the end of the function.
+
+extern int use(int);
+int glob1;
+int glob2;
+float globf;
+
+int foo(int i) {
+  // CHECK: define{{.*}}foo
+  if (i < 4) {
+    int j = i * 3;
+    if (glob1 > 3) {
+      float f = globf;
+      // CHECK: [[SSAVAL:%[a-z0-9]*]] = load float{{.*}}globf
+      j = f;
+      glob2 = j;
+      // CHECK: store{{.*}}glob2
+      // CHECK-NEXT: call void (...) @llvm.fake.use(float [[SSAVAL]])
+    }
+    glob1 = j;
+    // CHECK: store{{.*}}glob1
+    // CHECK-NEXT: call void (...) @llvm.fake.use(i32 %j.
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SLTozer wrote:

AFAICT, for tests that invoke `%clang_cc1`, `-fno-discard-value-names` is the default - for value names to be discarded, the driver needs to pass `-discard-value-names` to `-cc1`.

https://github.com/llvm/llvm-project/pull/110102