[llvm] [SelectionDAG] Salvage debuginfo when combining load and z|s ext instrs. (PR #188544)

[Orlando Cazalet-Hyams via llvm-commits]

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@@ -0,0 +1,61 @@
+; This test checks that after SelectionDAG runs, it preserves the debug info that is lost due to the DAGCombiner combining a load and a sext instruction, where the #dbg_value is pointing to the result of the load.
+; RUN: llc %s -mtriple=x86_64-unkown-linux -start-before=x86-isel -stop-after=x86-isel -o - | FileCheck %s --check-prefix=MIR
+; RUN: llc -O2 %s -start-before=x86-isel -mtriple=x86_64-unkown-linux --filetype=obj -o %t.o 
+; RUN: llvm-dwarfdump %t.o --name Idx | FileCheck %s --check-prefix=DUMP
+; RUN: llvm-dwarfdump %t.o --name Idx2 | FileCheck %s --check-prefix=DUMP2
+
+; MIR: ![[IDX:[0-9]+]] = !DILocalVariable(name: "Idx"
+; MIR: ![[IDX2:[0-9]+]] = !DILocalVariable(name: "Idx2"
+; MIR-LABEL: bb.0
+; MIR: %{{[0-9a-f]+}}{{.*}} = MOVSX64rm32 ${{.*}}, 1, $noreg, @GlobArr, $noreg, debug-instr-number [[INSTR_NUM:[0-9]+]]
+; MIR-NEXT: DBG_INSTR_REF ![[IDX]], !DIExpression(DW_OP_LLVM_arg, 0,  DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_LLVM_convert, 64, DW_ATE_signed, DW_OP_stack_value), dbg-instr-ref([[INSTR_NUM]], 0)
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OCHyams wrote:

Is the from/to the wrong way around here? Since we want to undo the effect of the ext for the variable location (opposite to usual salvaging, which wants to preserve the effect of an instruction that is deleted).

https://github.com/llvm/llvm-project/pull/188544