https://bugs.llvm.org/show_bug.cgi?id=38211
Bug ID: 38211
Summary: [RegisterScavenging] Scavenging results in "Using an
undefined physical register"
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: pauls...@linux.vnet.ibm.com
CC: llvm-bugs@lists.llvm.org
Created attachment 20569
--> https://bugs.llvm.org/attachment.cgi?id=20569&action=edit
reduced testcase (llc .mir input)
This test case involves an MVC (mem-2-mem move) between two spill slots:
MVC %stack.180, 0, 16, %stack.1, 0 :: (store 16 into %stack.180, align 8),
(load 16 from %stack.1, align 8)
PrologEpilogInserter eliminates these frame indexes, and while doing so (since
the offset from SP to %stack.1 is too big) a virtual register is created to
load SP + 4096:
%12:addr64bit = LAY $r15d, 4096, $noreg
Then, $r14d is scavenged for %12. This results in:
STG killed $r14d, $r15d, 168, $noreg :: (store 8 into %stack.329)
$r14d = LAY $r15d, 4096, $noreg
MVC $r15d, 1824, 16, killed $r14d, 592 :: (store 16 into %stack.180, align 8),
(load 16 from %stack.1, align 8)
$r14d = LG $r15d, 168, $noreg :: (load 8 from %stack.329)
It seems that $14d was actually not live, so it wouldn't have needed to be
spilled and restored:
*** Bad machine code: Using an undefined physical register ***
- function: main
- basic block: %bb.1 bb146 (0x2aa4cf914e8)
- instruction: STG killed $r14d, $r15d, 168, $noreg :: (store 8 into
%stack.329)
- operand 0: killed $r14d
LLVM ERROR: Found 1 machine code errors.
I am not sure exactly how this works, but it seems that
RegScavenger::scavengeRegisterBackwards() has somehow reported that $14d should
be spilled, while the MachineVerifier thinks that is not live.
bin/llc -mcpu=z13 ./tc_undefpreg_pei.mir -run-pass=prologepilog
-verify-machineinstrs
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