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
diff --git a/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp b/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
index c6b420f..9d6feaa 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
@@ -263,6 +263,7 @@
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 @@
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 @@
if (ExecBranchStack.empty()) {
if (skipIfDead(MI, *NextBB)) {
+ HaveSkipBlock = true;
NextBB = std::next(BI);
BE = MF.end();
- Next = MBB.end();
}
} else {
HaveKill = true;