[PATCH] D18601: AMDGPU/SI: Optimize adjacent s_nop instructions

[Tom Stellard via llvm-commits]

tstellarAMD created this revision.
tstellarAMD added a reviewer: arsenm.
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Herald added a subscriber: arsenm.

Use the operand for how long to wait. This is somewhat
distasteful, since it would be better to just emit s_nop
with the right argument in the first place. This would require
changing TII::insertNoop to emit N operands, which would be easy.
Slightly more problematic is the post-RA scheduler and hazard recognizer
represent nops as a single null node, and would require inventing
another way of representing N nops.

Patch by: Matt Arsenault

http://reviews.llvm.org/D18601

Files:
  lib/Target/AMDGPU/SIShrinkInstructions.cpp

Index: lib/Target/AMDGPU/SIShrinkInstructions.cpp
===================================================================
--- lib/Target/AMDGPU/SIShrinkInstructions.cpp
+++ lib/Target/AMDGPU/SIShrinkInstructions.cpp
@@ -213,6 +213,7 @@
     for (I = MBB.begin(); I != MBB.end(); I = Next) {
       Next = std::next(I);
       MachineInstr &MI = *I;
+      MachineInstr &NextMI = *Next;
 
       // Try to use S_MOVK_I32, which will save 4 bytes for small immediates.
       if (MI.getOpcode() == AMDGPU::S_MOV_B32) {
@@ -249,6 +250,29 @@
           }
         }
       }
+      // Combine adjacent s_nops to use the immediate operand encoding how long
+      // to wait.
+      //
+      // s_nop N
+      // s_nop M
+      //  =>
+      // s_nop (N + M)
+      if (MI.getOpcode() == AMDGPU::S_NOP &&
+          NextMI.getOpcode() == AMDGPU::S_NOP) {
+        // The instruction encodes the amount to wait with an offset of 1,
+        // i.e. 0 is wait 1 cycle. Convert both to cycles and then convert back
+        // after adding.
+        uint8_t Nop0 = MI.getOperand(0).getImm() + 1;
+        uint8_t Nop1 = NextMI.getOperand(0).getImm() + 1;
+
+        // Make sure we don't overflow the bounds.
+        if (Nop0 + Nop1 <= 8) {
+          NextMI.getOperand(0).setImm(Nop0 + Nop1 - 1);
+          MI.eraseFromParent();
+        }
+
+        continue;
+      }
 
       if (!TII->hasVALU32BitEncoding(MI.getOpcode()))
         continue;


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