[PATCH] D57718: [PPC] Adjust the computed branch offset for the possible shorter distance

[Guozhi Wei via Phabricator via llvm-commits]

Carrot created this revision.
Carrot added reviewers: echristo, kbarton, nemanjai.
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In file PPCBranchSelector.cpp we tend to over estimate code size due to large alignment and
inline assembly. Usually it causes larger computed branch offset, it is not big problem.
But sometimes it may also causes smaller computed branch offset
than actual branch offset. If the offset is close to the limit of
encoding, it may cause problem at run time.
Following is a simplified example.

             actual        estimated
             address        address
   ...
  bne Far      100            10c
  .p2align 4
  Near:        110            110
   ...
  Far:        8108           8108
  
  Actual offset:    0x8108 - 0x100 = 0x8008
  Computed offset:  0x8108 - 0x10c = 0x7ffc

The computed offset is at most ((1 << alignment) - 4) bytes smaller
 than actual offset. So we add this number to the offset for safety.


Repository:
  rL LLVM

https://reviews.llvm.org/D57718

Files:
  lib/Target/PowerPC/PPCBranchSelector.cpp


Index: lib/Target/PowerPC/PPCBranchSelector.cpp
===================================================================
--- lib/Target/PowerPC/PPCBranchSelector.cpp
+++ lib/Target/PowerPC/PPCBranchSelector.cpp
@@ -25,6 +25,7 @@
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Target/TargetMachine.h"
+#include <algorithm>
 using namespace llvm;
 
 #define DEBUG_TYPE "ppc-branch-select"
@@ -189,10 +190,39 @@
         } else {
           // Otherwise, add the size of the blocks between this block and the
           // dest to the number of bytes left in this block.
-          BranchSize = -MBBStartOffset;
+          unsigned StartBlock = MBB.getNumber();
+          BranchSize = BlockSizes[StartBlock].first - MBBStartOffset;
 
-          for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
+          unsigned MaxAlign = 2;
+          MaxAlign = std::max(MaxAlign, Dest->getAlignment());
+          for (unsigned i = StartBlock+1, e = Dest->getNumber(); i != e; ++i) {
             BranchSize += BlockSizes[i].first;
+            MaxAlign = std::max(MaxAlign,
+                                Fn.getBlockNumbered(i)->getAlignment());
+          }
+
+          // We tend to over estimate code size due to large alignment and
+          // inline assembly. Usually it causes larger computed branch offset.
+          // But sometimes it may also causes smaller computed branch offset
+          // than actual branch offset. If the offset is close to the limit of
+          // encoding, it may cause problem at run time.
+          // Following is a simplified example.
+          //
+          //              actual        estimated
+          //              address        address
+          //    ...
+          //   bne Far      100            10c
+          //   .p2align 4
+          //   Near:        110            110
+          //    ...
+          //   Far:        8108           8108
+          //
+          //   Actual offset:    0x8108 - 0x100 = 0x8008
+          //   Computed offset:  0x8108 - 0x10c = 0x7ffc
+          //
+          // The computed offset is at most ((1 << alignment) - 4) bytes smaller
+          // than actual offset. So we add this number to the offset for safety.
+          BranchSize += (1 << MaxAlign) - 4;
         }
 
         // If this branch is in range, ignore it.


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