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

1 files changed, 40 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir b/llvm/test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir
new file mode 100644
index 00000000000..bd5f296affb
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir
@@ -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