[PATCH] D107810: [ARM] Improve detection of fallthough when aligning blocks

[Dave Green via Phabricator via llvm-commits]

dmgreen updated this revision to Diff 370927.
dmgreen added a comment.

Check LOB instructions specifically.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107810/new/

https://reviews.llvm.org/D107810

Files:
  llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
  llvm/test/CodeGen/Thumb2/aligned-nonfallthrough.ll


Index: llvm/test/CodeGen/Thumb2/aligned-nonfallthrough.ll
===================================================================
--- llvm/test/CodeGen/Thumb2/aligned-nonfallthrough.ll
+++ llvm/test/CodeGen/Thumb2/aligned-nonfallthrough.ll
@@ -14,7 +14,6 @@
 ; CHECK-NEXT:    ldr r2, [r0], #4
 ; CHECK-NEXT:    add r1, r2
 ; CHECK-NEXT:    le lr, .LBB0_1
-; CHECK-NEXT:    .p2align 2
 ; CHECK-NEXT:  @ %bb.2: @ %for.cond.cleanup
 ; CHECK-NEXT:    mov r0, r1
 ; CHECK-NEXT:    pop {r7, pc}
@@ -54,7 +53,6 @@
 ; CHECK-NEXT:    ldr r2, [r12], #4
 ; CHECK-NEXT:    smlal r0, r3, r2, r1
 ; CHECK-NEXT:    le lr, .LBB1_2
-; CHECK-NEXT:    .p2align 2
 ; CHECK-NEXT:  @ %bb.3: @ %for.cond.cleanup
 ; CHECK-NEXT:    mov r1, r3
 ; CHECK-NEXT:    pop {r7, pc}
Index: llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -18,6 +18,7 @@
 #include "ARMMachineFunctionInfo.h"
 #include "ARMSubtarget.h"
 #include "MCTargetDesc/ARMBaseInfo.h"
+#include "MVETailPredUtils.h"
 #include "Thumb2InstrInfo.h"
 #include "Utils/ARMBaseInfo.h"
 #include "llvm/ADT/DenseMap.h"
@@ -340,12 +341,12 @@
 // Align blocks where the previous block does not fall through. This may add
 // extra NOP's but they will not be executed. It uses the PrefLoopAlignment as a
 // measure of how much to align, and only runs at CodeGenOpt::Aggressive.
-static bool AlignBlocks(MachineFunction *MF) {
+static bool AlignBlocks(MachineFunction *MF, const ARMSubtarget *STI) {
   if (MF->getTarget().getOptLevel() != CodeGenOpt::Aggressive ||
       MF->getFunction().hasOptSize())
     return false;
 
-  auto *TLI = MF->getSubtarget().getTargetLowering();
+  auto *TLI = STI->getTargetLowering();
   const Align Alignment = TLI->getPrefLoopAlignment();
   if (Alignment < 4)
     return false;
@@ -357,7 +358,26 @@
       Changed = true;
       MBB.setAlignment(Alignment);
     }
+
     PrevCanFallthough = MBB.canFallThrough();
+
+    // For LOB's, the ARMLowOverheadLoops pass may remove the unconditional
+    // branch later in the pipeline.
+    if (STI->hasLOB()) {
+      for (const auto &MI : reverse(MBB.terminators())) {
+        dbgs() << "MI: " << MI << "\n";
+        if (MI.getOpcode() == ARM::t2B &&
+            MI.getOperand(0).getMBB() == MBB.getNextNode())
+          continue;
+        if (isLoopStart(MI) || MI.getOpcode() == ARM::t2LoopEnd ||
+            MI.getOpcode() == ARM::t2LoopEndDec) {
+          PrevCanFallthough = true;
+          break;
+        }
+        // Any other terminator - nothing to do
+        break;
+      }
+    }
   }
 
   return Changed;
@@ -406,7 +426,7 @@
   }
 
   // Align any non-fallthrough blocks
-  MadeChange |= AlignBlocks(MF);
+  MadeChange |= AlignBlocks(MF, STI);
 
   // Perform the initial placement of the constant pool entries.  To start with,
   // we put them all at the end of the function.


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