[PATCH] D28351: AMDGPU: Remove spurious out branches after a kill

[Samuel Pitoiset via Phabricator via llvm-commits]

hakzsam updated this revision to Diff 84531.
hakzsam added a comment.

v3: - only use CHECK_LABEL for the start of a function

- check there is no spurious branch after the conditional branch
- use an else-if to avoid potential double-erasing


https://reviews.llvm.org/D28351

Files:
  lib/Target/AMDGPU/SIInsertSkips.cpp
  test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir


Index: test/CodeGen/AMDGPU/insert-skips-kill-uncond.mir
===================================================================
--- /dev/null
+++ 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
+
+--- |
+  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
Index: lib/Target/AMDGPU/SIInsertSkips.cpp
===================================================================
--- lib/Target/AMDGPU/SIInsertSkips.cpp
+++ lib/Target/AMDGPU/SIInsertSkips.cpp
@@ -251,6 +251,7 @@
        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.
@@ -278,8 +279,14 @@
       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: {
@@ -288,9 +295,9 @@
 
         if (ExecBranchStack.empty()) {
           if (skipIfDead(MI, *NextBB)) {
+            HaveSkipBlock = true;
             NextBB = std::next(BI);
             BE = MF.end();
-            Next = MBB.end();
           }
         } else {
           HaveKill = true;


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