ELF: Try to create last thunk section at ThunkSectionSpacing bytes before the end.

Now that we have the ability to create short thunks, it is beneficial
for thunk sections to be surrounded by ThunkSectionSpacing bytes
of code on both sides in order to increase the likelihood that the
distance from the thunk to the target will be sufficiently small to
allow for the creation of a short thunk. This is currently the case
for most thunks that we create, except for the last one, which could,
depending on the size of the output section, potentially appear near
the end and therefore have a relatively small amount of code after it.

This patch moves the last thunk section to ThunkSectionSpacing bytes
before the end of the output section, as long as the section is larger
than 2*ThunkSectionSpacing bytes. It reduces the size of Chromium
for Android's .text section by 32KB.

Differential Revision: https://reviews.llvm.org/D44966

llvm-svn: 328889

diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index d32b584..bc4f4b2 100644 (file)
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -1218,17 +1218,30 @@ ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS) {
 //
 // We follow a simple but conservative heuristic to place ThunkSections at
 // offsets that are multiples of a Target specific branch range.
-// For an InputSectionRange that is smaller than the range, a single
+// For an InputSectionDescription that is smaller than the range, a single
 // ThunkSection at the end of the range will do.
+//
+// For an InputSectionDescription that is more than twice the size of the range,
+// we place the last ThunkSection at range bytes from the end of the
+// InputSectionDescription in order to increase the likelihood that the
+// distance from a thunk to its target will be sufficiently small to
+// allow for the creation of a short thunk.
 void ThunkCreator::createInitialThunkSections(
     ArrayRef<OutputSection *> OutputSections) {
   forEachInputSectionDescription(
       OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
         if (ISD->Sections.empty())
           return;
+        uint32_t ISDBegin = ISD->Sections.front()->OutSecOff;
+        uint32_t ISDEnd =
+            ISD->Sections.back()->OutSecOff + ISD->Sections.back()->getSize();
+        uint32_t LastThunkLowerBound = -1;
+        if (ISDEnd - ISDBegin > Target->ThunkSectionSpacing * 2)
+          LastThunkLowerBound = ISDEnd - Target->ThunkSectionSpacing;
+
         uint32_t ISLimit;
-        uint32_t PrevISLimit = ISD->Sections.front()->OutSecOff;
-        uint32_t ThunkUpperBound = PrevISLimit + Target->ThunkSectionSpacing;
+        uint32_t PrevISLimit = ISDBegin;
+        uint32_t ThunkUpperBound = ISDBegin + Target->ThunkSectionSpacing;
 
         for (const InputSection *IS : ISD->Sections) {
           ISLimit = IS->OutSecOff + IS->getSize();
@@ -1236,6 +1249,8 @@ void ThunkCreator::createInitialThunkSections(
             addThunkSection(OS, ISD, PrevISLimit);
             ThunkUpperBound = PrevISLimit + Target->ThunkSectionSpacing;
           }
+          if (ISLimit > LastThunkLowerBound)
+            break;
           PrevISLimit = ISLimit;
         }
         addThunkSection(OS, ISD, ISLimit);

diff --git a/lld/test/ELF/arm-thumb-mix-range-thunk-os.s b/lld/test/ELF/arm-thumb-mix-range-thunk-os.s
index beff414..b5db256 100644 (file)
--- a/lld/test/ELF/arm-thumb-mix-range-thunk-os.s
+++ b/lld/test/ELF/arm-thumb-mix-range-thunk-os.s
@@ -11,7 +11,7 @@
 // RUN: llvm-objdump -d %t2 -start-address=35651584 -stop-address=35651590 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK6 %s
 // RUN: llvm-objdump -d %t2 -start-address=36700160 -stop-address=36700168 -triple=armv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK7 %s
 // RUN: llvm-objdump -d %t2 -start-address=48234500 -stop-address=48234512 -triple=armv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK8 %s
-// RUN: llvm-objdump -d %t2 -start-address=63963140 -stop-address=63963160 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK9 %s
+// RUN: llvm-objdump -d %t2 -start-address=53477380 -stop-address=53477392 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK9 %s
 // RUN: llvm-objdump -d %t2 -start-address=68157440 -stop-address=68157452 -triple=armv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK10 %s
 // RUN: llvm-objdump -d %t2 -start-address=69206016 -stop-address=69206024 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK11 %s
 
@@ -155,6 +155,13 @@ _start:
  ARMFUNCTION 48
  THUMBFUNCTION 49
  ARMFUNCTION 50
+// Expect precreated Thunk Section here
+// CHECK9: __Thumbv7ABSLongThunk_afunc34:
+// CHECK9-NEXT:  3300004:       40 f2 00 0c     movw    r12, #0
+// CHECK9-NEXT:  3300008:       c0 f2 30 2c     movt    r12, #560
+// CHECK9-NEXT:  330000c:       60 47   bx      r12
+// CHECK9: __Thumbv7ABSLongThunk_tfunc35:
+// CHECK9-NEXT:  330000e:       ff f4 f7 97     b.w     #-15728658 <tfunc35>
  THUMBFUNCTION 51
  ARMFUNCTION 52
  THUMBFUNCTION 53
@@ -165,15 +172,6 @@ _start:
  ARMFUNCTION 58
  THUMBFUNCTION 59
  ARMFUNCTION 60
-// Expect precreated Thunk Section here
-// CHECK9: __Thumbv7ABSLongThunk_afunc34:
-// CHECK9-NEXT:  3d00004:       40 f2 00 0c     movw    r12, #0
-// CHECK9-NEXT:  3d00008:       c0 f2 30 2c     movt    r12, #560
-// CHECK9-NEXT:  3d0000c:       60 47   bx      r12
-// CHECK9: __Thumbv7ABSLongThunk_tfunc35:
-// CHECK9-NEXT:  3d0000e:       40 f2 01 0c     movw    r12, #1
-// CHECK9-NEXT:  3d00012:       c0 f2 40 2c     movt    r12, #576
-// CHECK9-NEXT:  3d00016:       60 47   bx      r12
  THUMBFUNCTION 61
  ARMFUNCTION 62
  THUMBFUNCTION 63
@@ -191,5 +189,5 @@ _start:
  bl tfunc35
 // CHECK11: tfunc65:
 // CHECK11:  4200000:   70 47   bx      lr
-// CHECK11-NEXT:  4200002:      ff f6 ff f7     bl      #-5242882
-// CHECK11-NEXT:  4200006:      00 f7 02 f0     bl      #-5242876
+// CHECK11-NEXT:  4200002:      ff f4 ff d7     bl      #-15728642
+// CHECK11-NEXT:  4200006:      00 f5 02 d0     bl      #-15728636

diff --git a/lld/test/ELF/arm-thumb-range-thunk-os.s b/lld/test/ELF/arm-thumb-range-thunk-os.s
index ae8c704..182b18d 100644 (file)
--- a/lld/test/ELF/arm-thumb-range-thunk-os.s
+++ b/lld/test/ELF/arm-thumb-range-thunk-os.s
@@ -9,8 +9,8 @@
 // RUN: llvm-objdump -d %t2 -start-address=4194304 -stop-address=4194310 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK4 %s
 // RUN: llvm-objdump -d %t2 -start-address=16777216 -stop-address=16777270 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK5 %s
 // RUN: llvm-objdump -d %t2 -start-address=17825792 -stop-address=17825808 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK6 %s
-// RUN: llvm-objdump -d %t2 -start-address=31457280 -stop-address=31457286 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK7 %s
-// RUN: llvm-objdump -d %t2 -start-address=32505860 -stop-address=32505880 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK8 %s
+// RUN: llvm-objdump -d %t2 -start-address=20971524 -stop-address=20971532 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK7 %s
+// RUN: llvm-objdump -d %t2 -start-address=31457280 -stop-address=31457286 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK8 %s
 // RUN: llvm-objdump -d %t2 -start-address=35651584 -stop-address=35651594 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK9 %s
 // RUN: llvm-objdump -d %t2 -start-address=36700160 -stop-address=36700170 -triple=thumbv7a-linux-gnueabihf | FileCheck -check-prefix=CHECK10 %s
 
@@ -105,6 +105,11 @@ _start:
  FUNCTION 16
  FUNCTION 17
  FUNCTION 18
+// Expect another precreated thunk section here
+// CHECK7: __Thumbv7ABSLongThunk_tfunc15:
+// CHECK7-NEXT:  1400004:       ff f4 fc bf     b.w     #-3145736 <tfunc15>
+// CHECK7: __Thumbv7ABSLongThunk_tfunc16:
+// CHECK7-NEXT:  1400008:       ff f5 fa bf     b.w     #-2097164 <tfunc16>
  FUNCTION 19
  FUNCTION 20
  FUNCTION 21
@@ -117,17 +122,12 @@ _start:
  FUNCTION 28
 // tfunc02 is > 16Mb away, expect range extension thunks in precreated thunk
 // section
-// CHECK7:  tfunc28:
-// CHECK7-NEXT:  1e00000:       70 47   bx      lr
-// CHECK7-NEXT:  1e00002:       00 f6 0d 90     b.w     #-14680038 <__Thumbv7ABSLongThunk_tfunc02>
+// CHECK8:  tfunc28:
+// CHECK8-NEXT:  1e00000:       70 47   bx      lr
+// CHECK8-NEXT:  1e00002:       00 f6 0d 90     b.w     #-14680038 <__Thumbv7ABSLongThunk_tfunc02>
 
  b.w tfunc02
  FUNCTION 29
-// Expect another precreated thunk section here
-// CHECK8: __Thumbv7ABSLongThunk_tfunc15:
-// CHECK8-NEXT:  1f00004:       ff f5 fc 97     b.w     #-14680072 <tfunc15>
-// CHECK8: __Thumbv7ABSLongThunk_tfunc16:
-// CHECK8-NEXT:  1f00008:       ff f6 fa 97     b.w     #-13631500 <tfunc16>
  FUNCTION 30
  FUNCTION 31
  FUNCTION 32
@@ -137,13 +137,13 @@ _start:
  bl tfunc16
 // CHECK9: tfunc32:
 // CHECK9:  2200000:    70 47   bx      lr
-// CHECK9-NEXT:  2200002:       ff f4 ff ff     bl      #-3145730
-// CHECK9-NEXT:  2200006:       ff f4 ff ff     bl      #-3145730
+// CHECK9-NEXT:  2200002:       ff f5 ff d7     bl      #-14680066
+// CHECK9-NEXT:  2200006:       ff f5 ff d7     bl      #-14680066
 
  FUNCTION 33
  bl tfunc15
  bl tfunc16
 // CHECK10: tfunc33:
 // CHECK10:  2300000:   70 47   bx      lr
-// CHECK10-NEXT:  2300002:      ff f7 ff f7     bl      #-4194306
-// CHECK10-NEXT:  2300006:      ff f7 ff f7     bl      #-4194306
+// CHECK10-NEXT:  2300002:      ff f4 ff d7     bl      #-15728642
+// CHECK10-NEXT:  2300006:      ff f4 ff d7     bl      #-15728642