[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,223 @@
+# Prevent the machine scheduler from moving instructions past FAKE_USE.
+# RUN: llc -run-pass machine-scheduler -debug-only=machine-scheduler 2>&1 -o - %s | FileCheck %s
+# REQUIRES: asserts
+#
+# We make sure that, beginning with the first FAKE_USE instruction,
+# no changes to the sequence of instructions are undertaken by the
+# scheduler. We don't bother to check that the order of the FAKE_USEs
+# remains the same. They should, but it is irrelevant.
+#
+# CHECK: ********** MI Scheduling **********
+# CHECK-NEXT: foo:%bb.0 entry
+# CHECK-NEXT:   From: %0:gr64 = COPY $rdi
+# CHECK-NEXT:     To: FAKE_USE %5:gr64
+# CHECK-NEXT:  RegionInstrs: 7
+#
+# CHECK: ********** MI Scheduling **********
+# CHECK-NEXT: bar:%bb.0 entry
+# CHECK-NEXT:   From: %0:gr64 = COPY $rdi
+# CHECK-NEXT:     To: RET 0, killed $rax
+# CHECK-NEXT:  RegionInstrs: 7
+#
+--- |
+  ; ModuleID = 'test.ll'
+  source_filename = "test.ll"
+  target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+  
+  @glb = common dso_local local_unnamed_addr global [100 x i32] zeroinitializer, align 16
+  
+  define dso_local i64 @foo(ptr %p) local_unnamed_addr {
+  entry:
+    %0 = load i32, ptr @glb, align 16, !tbaa !2
+    store i32 %0, ptr %p, align 4, !tbaa !2
+    %conv = sext i32 %0 to i64
+    %1 = load i32, ptr getelementptr inbounds ([100 x i32], ptr @glb, i64 0, i64 1), align 4, !tbaa !2
+    %arrayidx1 = getelementptr inbounds i32, ptr %p, i64 1
+    store i32 %1, ptr %arrayidx1, align 4, !tbaa !2
+    %conv2 = sext i32 %1 to i64
+    %add3 = add nsw i64 %conv2, %conv
+    notail call void (...) @llvm.fake.use(i64 %add3)
+    notail call void (...) @llvm.fake.use(i32 %1)
+    notail call void (...) @llvm.fake.use(i32 %0)
+    notail call void (...) @llvm.fake.use(ptr %p)
+    ret i64 %add3
+  }
+  
+  define dso_local i64 @bar(ptr %p) local_unnamed_addr {
+  entry:
+    %0 = load i32, ptr @glb, align 16, !tbaa !2
+    store i32 %0, ptr %p, align 4, !tbaa !2
+    %conv = sext i32 %0 to i64
+    %1 = load i32, ptr getelementptr inbounds ([100 x i32], ptr @glb, i64 0, i64 1), align 4, !tbaa !2
+    %arrayidx1 = getelementptr inbounds i32, ptr %p, i64 1
+    store i32 %1, ptr %arrayidx1, align 4, !tbaa !2
+    %conv2 = sext i32 %1 to i64
+    %add3 = add nsw i64 %conv2, %conv
+    ret i64 %add3
+  }
+  
+  ; Function Attrs: nounwind
+  declare void @llvm.fake.use(...) #0
+  
+  ; Function Attrs: nocallback nofree nosync nounwind willreturn
+  declare void @llvm.stackprotector(ptr, ptr) #1
+  
+  attributes #0 = { nounwind }
+  attributes #1 = { nocallback nofree nosync nounwind willreturn }
+  
+  !llvm.module.flags = !{!0}
+  !llvm.ident = !{!1}
+  
+  !0 = !{i32 1, !"wchar_size", i32 4}
+  !1 = !{!"clang version 9.0.0"}
+  !2 = !{!3, !3, i64 0}
+  !3 = !{!"int", !4, i64 0}
+  !4 = !{!"omnipotent char", !5, i64 0}
+  !5 = !{!"Simple C/C++ TBAA"}
+
+...
+---
+name:            foo
+alignment:       16
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+callsEHReturn:   false
+callsUnwindInit: false
+hasEHCatchret:   false
+hasEHScopes:     false
+hasEHFunclets:   false
+isOutlined:      false
+debugInstrRef:   true
+failsVerification: false
+tracksDebugUserValues: false
+registers:
+  - { id: 0, class: gr64, preferred-register: '' }
+  - { id: 1, class: gr64_with_sub_8bit, preferred-register: '' }
+  - { id: 2, class: gr32, preferred-register: '' }
+  - { id: 3, class: gr64_with_sub_8bit, preferred-register: '' }
+  - { id: 4, class: gr32, preferred-register: '' }
+  - { id: 5, class: gr64, preferred-register: '' }
+liveins:
+  - { reg: '$rdi', virtual-reg: '%0' }
+frameInfo:
+  isFrameAddressTaken: false
----------------
arsenm wrote:

Can remove most of this, -simplify-mir helps 

https://github.com/llvm/llvm-project/pull/86149