[lld] [lld][WebAssembly] Drop llvm_cov sections when llvm_prf segments are discarded (PR #172023)

[Sam Clegg via llvm-commits]

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@@ -0,0 +1,28 @@
+; RUN: llc -filetype=obj %s -o %t.instr-prof.o
+; RUN: llc -filetype=obj %p/Inputs/malformed-prf1.ll -o %t.malformed-prf1.o
+; RUN: llc -filetype=obj %p/Inputs/malformed-prf2.ll -o %t.malformed-prf2.o
+; RUN: wasm-ld -o %t.wasm %t.instr-prof.o %t.malformed-prf1.o --start-lib %t.malformed-prf2.o --end-lib --gc-sections --no-entry
+; RUN: llvm-cov export --object %t.wasm --empty-profile
+
+; Every covfun record holds a hash of its symbol name, and llvm-cov will exit fatally if
+; it can't resolve that hash back to an entry in the binary's `__llvm_prf_names` linker section.
+;
+; WASM stores `__llvm_covfun` in custom section, while `__llvm_prf_names` is stored in the DATA section.
+; The former not be GC, whereas the latter may be GC, causing llvm-cov execution to fail.
+;
+; Now, __llvm_covfun and __llvm_covmap will be dropped if __llvm_prf_names is discarded.
+
+; ModuleID = 'instr-prof.c'
+source_filename = "instr-prof.c"
+target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-i128:128-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-unknown"
+
+ at __llvm_profile_runtime = hidden global i32 0, align 4
+
+!llvm.module.flags = !{!0, !1}
+!llvm.ident = !{!2}
+
+!0 = !{i32 2, !"EnableValueProfiling", i32 0}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{!"clang version 21.1.6"}
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sbc100 wrote:

The reason is that we want to keep the system-under-test as small as possible.  If we used `clang -S -emit-llvm` then changed to clang could effect lld tests, which would make them way less stable, and make test a failure way more involved to debug.

Even using llvm ir here is kind sub-optimal because it means the tests depends on large parts of llvm includes instruction selelction.    Tests written in `.s` format depend only the MC layer and are far more stable/simple.

https://github.com/llvm/llvm-project/pull/172023