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GlobalISel: Disallow undef generic virtual register uses

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With an undef operand, it's possible for getVRegDef to fail and return
null. This is an edge case very little code bothered to
consider. Proper gMIR should use G_IMPLICIT_DEF instead.

I initially tried to apply this restriction to all SSA MIR, so then
getVRegDef would never fail anywhere. However, ProcessImplicitDefs
does technically run while the function is in SSA. ProcessImplicitDefs
and DetectDeadLanes would need to either move, or a new pseudo-SSA
type of function property would need to be introduced.
This commit is contained in:
Matt Arsenault 2020-06-28 10:06:58 -04:00
parent f7dc4642fe
commit 58a9482211
2 changed files with 34 additions and 0 deletions
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9
lib/CodeGen/MachineVerifier.cpp
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@ -1695,6 +1695,15 @@ MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
if (!RC) {
// This is a generic virtual register.
// Do not allow undef uses for generic virtual registers. This ensures
// getVRegDef can never fail and return null on a generic register.
//
// FIXME: This restriction should probably be broadened to all SSA
// MIR. However, DetectDeadLanes/ProcessImplicitDefs technically still
// run on the SSA function just before phi elimination.
if (MO->isUndef())
report("Generic virtual register use cannot be undef", MO, MONum);
// If we're post-Select, we can't have gvregs anymore.
if (isFunctionSelected) {
report("Generic virtual register invalid in a Selected function",

25
test/MachineVerifier/generic-vreg-undef-use.mir Normal file
View File

@ -0,0 +1,25 @@
# RUN: not --crash llc -march=aarch64 -o /dev/null -run-pass=none -verify-machineinstrs %s 2>&1 | FileCheck %s
# REQUIRES: aarch64-registered-target
# Undef uses are illegal for generic vregs.
---
name: test_undef_use
liveins:
body: |
bb.0:
%0:_(s32) = G_CONSTANT i32 0
; Test generic instruction
; CHECK: *** Bad machine code: Generic virtual register use cannot be undef ***
G_STORE %0, undef %1:_(p0) :: (store 4)
; Make sure this fails on a post-isel generic instruction.
; CHECK: *** Bad machine code: Generic virtual register use cannot be undef ***
$x0 = COPY undef %2:_(s64)
; Make sure this fails with a target instruction
; CHECK: *** Bad machine code: Generic virtual register use cannot be undef ***
RET_ReallyLR implicit $x0, implicit undef %3:_(s32)
...