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AMDGPU: Remove spurious out branches after a kill 

The sequence like this:
  v_cmpx_le_f32_e32 vcc, 0, v0
  s_branch BB0_30
  s_cbranch_execnz BB0_30
  ; BB#29:
  exp null off, off, off, off done vm
  s_endpgm
  BB0_30:
  ; %endif110

is likely wrong. The s_branch instruction will unconditionally jump
to BB0_30 and the skip block (exp done + endpgm) inserted for
performing the kill instruction will never be executed. This results
in a GPU hang with Star Ruler 2.

The s_branch instruction is added during the "Control Flow Optimizer"
pass which seems to re-organize the basic blocks, and we assume
that SI_KILL_TERMINATOR is always the last instruction inside a
basic block. Thus, after inserting a skip block we just go to the
next BB without looking at the subsequent instructions after the
kill, and the s_branch op is never removed.

Instead, we should remove the unconditional out branches and let
skip the two instructions if the exec mask is non-zero.

This patch fixes the GPU hang and doesn't introduce any regressions
with "make check".

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99019

Patch by Samuel Pitoiset <samuel.pitoiset@gmail.com>

llvm-svn: 292985
This commit is contained in:
Matt Arsenault 2017-01-24 22:18:39 +00:00
parent f1cf0278e8
commit bf67cf7e4b
2 changed files with 49 additions and 2 deletions
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11
llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
View File

@ -263,6 +263,7 @@ bool SIInsertSkips::runOnMachineFunction(MachineFunction &MF) {
BI != BE; BI = NextBB) {
NextBB = std::next(BI);
MachineBasicBlock &MBB = *BI;
bool HaveSkipBlock = false;
if (!ExecBranchStack.empty() && ExecBranchStack.back() == &MBB) {
// Reached convergence point for last divergent branch.
@ -290,8 +291,14 @@ bool SIInsertSkips::runOnMachineFunction(MachineFunction &MF) {
case AMDGPU::S_BRANCH:
// Optimize out branches to the next block.
// FIXME: Shouldn't this be handled by BranchFolding?
if (MBB.isLayoutSuccessor(MI.getOperand(0).getMBB()))
if (MBB.isLayoutSuccessor(MI.getOperand(0).getMBB())) {
MI.eraseFromParent();
} else if (HaveSkipBlock) {
// Remove the given unconditional branch when a skip block has been
// inserted after the current one and let skip the two instructions
// performing the kill if the exec mask is non-zero.
MI.eraseFromParent();
}
break;
case AMDGPU::SI_KILL_TERMINATOR:
@ -300,9 +307,9 @@ bool SIInsertSkips::runOnMachineFunction(MachineFunction &MF) {
if (ExecBranchStack.empty()) {
if (skipIfDead(MI, *NextBB)) {
HaveSkipBlock = true;
NextBB = std::next(BI);
BE = MF.end();
Next = MBB.end();
}
} else {
HaveKill = true;

40
llvm/test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir Normal file
View File

@ -0,0 +1,40 @@
# RUN: llc -march=amdgcn -mcpu=polaris10 -run-pass si-insert-skips -amdgpu-skip-threshold=1 %s -o - | FileCheck %s
# https://bugs.freedesktop.org/show_bug.cgi?id=99019
--- |
define amdgpu_ps void @kill_uncond_branch() {
ret void
}
...
---
# CHECK-LABEL: name: kill_uncond_branch
# CHECK: bb.0:
# CHECK: S_CBRANCH_VCCNZ %bb.1, implicit %vcc
# CHECK: bb.1:
# CHECK: V_CMPX_LE_F32_e32
# CHECK-NEXT: S_CBRANCH_EXECNZ %bb.2, implicit %exec
# CHECK: bb.3:
# CHECK-NEXT: EXP_DONE
# CHECK: S_ENDPGM
# CHECK: bb.2:
# CHECK: S_ENDPGM
name: kill_uncond_branch
body: |
bb.0:
successors: %bb.1
S_CBRANCH_VCCNZ %bb.1, implicit %vcc
bb.1:
successors: %bb.2
%vgpr0 = V_MOV_B32_e32 0, implicit %exec
SI_KILL_TERMINATOR %vgpr0, implicit-def %exec, implicit-def %vcc, implicit %exec
S_BRANCH %bb.2
bb.2:
S_ENDPGM