https://bugs.llvm.org/show_bug.cgi?id=41801
Bug ID: 41801
Summary: [ISel] Unused instructions referenced in debug data
removed at O0
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: stephen.to...@sony.com
CC: llvm-bugs@lists.llvm.org
Using LLVM/clang r360138 and the command line "-c -O0 -x ir" on the IR file
produced by the commands below, the debug data corresponding to "nose.b" and
"boot" is not present in the output object file. The issue appears to occur
during instruction selection, where the following register (which is only used
in debug expressions)is not present in the first pass to output MIR, either
"X86 DAG->DAG Instruction Selection" or "Expand ISel Pseudo-instructions"
depending on platform. This causes debug information to be lost due to an
optimization when -O0 is enabled.
To reproduce:
$ clang -v
clang version 9.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\dev\llvm-project\build\Debug\bin
$ cat reproduce.cpp
struct chin {
int a;
int b;
};
int foo(struct chin nose, int more) {
int shoe = nose.a;
int boot = nose.b;
return more;
}
$ clang reproduce.cpp -o - -S -emit-llvm -g -O0 -Xclang -disable-O0-optnone |
opt -sroa -early-cse -S > reproduce.ll
The point at which the debug information for "boot" is lost can then be seen in
the output of the command:
$ clang reproduce.ll -o - -c -g -O0 -x ir -mllvm -print-after-all 2>&1
>/dev/null
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