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

[Stephen Tozer via llvm-commits]

================
@@ -0,0 +1,112 @@
+; REQUIRES: object-emission
+
+; Make sure the fake use of 'b' at the end of 'foo' causes location information for 'b'
+; to extend all the way to the end of the function.
+; FIXME: We need to use `-debugger-tune=sce` to disable entry values, since otherwise
+; %b will be preserved as one; there doesn't seem to be another way to disable entry
+; values (--debug-entry-values is only used to force-enable them, not disable them).
+
+; RUN: %llc_dwarf -O2 -debugger-tune=sce -filetype=obj -dwarf-linkage-names=Abstract < %s | llvm-dwarfdump --debug-info --debug-line -v - -o %t
+; RUN: %python %p/Inputs/check-fake-use.py %t
+; RUN: %llc_dwarf -O2 -debugger-tune=sce --global-isel=1 -mtriple=aarch64--linux-gnu -filetype=obj -dwarf-linkage-names=Abstract < %s | llvm-dwarfdump --debug-info --debug-line -v - -o %t
+; RUN: %python %p/Inputs/check-fake-use.py %t
+; RUN: sed -e 's,call void (...) @llvm.fake.use,;,' %s | %llc_dwarf - -O2 -debugger-tune=sce -filetype=obj -dwarf-linkage-names=Abstract | llvm-dwarfdump --debug-info --debug-line -v - -o %t
+; RUN: not %python %p/Inputs/check-fake-use.py %t
----------------
SLTozer wrote:

Perhaps - it didn't occur to me because we _probably_ shouldn't see any other fail state emerge as a result of just turning off extend-lifetimes, but no harm in making it more specific!

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