For example, given code like this:
int local = value();
callee(local);
return local;
resulting in assembly like:
movl %eax, %ebx // %eax = local
movl %eax, %edi
callq callee@PLT
the call site value generation did not understand that the value of
local is stored in the callee-saved %ebx during the call, and did not
emit any call site value. This patch addresses that, resulting in:
DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg5 RDI)
DW_AT_call_value (DW_OP_breg3 RBX+0)
This code does not keep track if any succeeding instructions save
registers, meaning that it fails to emit a call site value for a case
like this:
movq %rax, %rdi
movq %rax, %rbx
callq callee@PLT
The test case dbgcall-site-preserved-clobbered.mir has been added for
that. This should be rather easy to address, but can be done in a
follow-up patch.
Building a clang-21 RelWithDebInfo binary for x86-64 without/with this
patch results in a ~1.8% increase of the number of call site parameter
entries with a location (from 1792876 to 1825718). This also reduces the
number of call site parameter entries using DW_OP_entry_value locations,
which are not guaranteed to be printable as they require the frame above
to provide a call site value for that parameter, from 57718 to 34871.
Fixes#43464.
30e399a675