[PATCH] D61610: [PPC64] implement Thunk Section Spacing

[Alfredo Dal'Ava Júnior via Phabricator via llvm-commits]

adalava updated this revision to Diff 198480.
adalava edited the summary of this revision.
adalava added a comment.

- add space reservation, add comments and improve test


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61610/new/

https://reviews.llvm.org/D61610

Files:
  lld/ELF/Arch/PPC64.cpp
  lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
  lld/test/ELF/ppc64-branch-thunkspacing.s


Index: lld/test/ELF/ppc64-branch-thunkspacing.s
===================================================================
--- lld/test/ELF/ppc64-branch-thunkspacing.s
+++ lld/test/ELF/ppc64-branch-thunkspacing.s
@@ -1,20 +1,21 @@
 # REQUIRES: ppc
 
-# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-freebsd13.0 %s -o %t
-# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-freebsd13.0 %S/Inputs/ppc64-far-branch-rangethunk.s -o %tfar
-# RUN: ld.lld  %t %tfar -o %t2 2>&1
-# RUN: llvm-objdump -d -triple=powerpc64-unknown-freebsd13.0 %t2 | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-freebsd13.0 %s -o %t.o
+# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-freebsd13.0 %S/Inputs/ppc64-far-branch-rangethunk.s -o %tfar.o
+# RUN: ld.lld  %t.o %tfar.o -o %t2
+# RUN: llvm-objdump -d %t2 | FileCheck %s
+
+.section .init,"ax", at progbits
+.align  2
+.globl  _start
+.type   _start, at function
 
-        .section .init,"ax", at progbits
-        .align  2
-        .globl  _start
-        .type   _start, at function
 _start:
-        stwu 1,-16(1)
-        mflr 0
-        stw 31,12(1)
-        stw 0,20(1)
-        mr 31,1
+	stwu 1,-16(1)
+	mflr 0
+	stw 31,12(1)
+	stw 0,20(1)
+	mr 31,1
 	bl  too_far1
 
 
Index: lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
===================================================================
--- lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
+++ lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
@@ -1,4 +1,6 @@
 
+.section .init,"ax", at progbits,unique,2
+.align 2
 .global too_far1
 .type too_far1,%function
 
Index: lld/ELF/Arch/PPC64.cpp
===================================================================
--- lld/ELF/Arch/PPC64.cpp
+++ lld/ELF/Arch/PPC64.cpp
@@ -927,8 +927,22 @@
 }
 
 uint32_t PPC64::getThunkSectionSpacing() const {
-  // REL14 range
-  return 0x8000;
+  // PowerPC64 has the following basic branch instructions:
+  //
+  //      - b[l,a]  PPC64_REL24 range [33,554,432...33,554,428]
+  //      - bc[l,a] PPC64_REL14 range [-32,768...32764]
+  //
+  // We take the most strict range and intentionally use a lower
+  // size than the maximum branch range so the end of the ThunkSection
+  // is more likely to be within range of the branch instruction that 
+  // is furthest away.
+  //
+  // Reducing it in 0x1000 allow creating 256 16 byte Thunks at any 
+  // offset in a ThunkSection without risk of a branch to one of the 
+  // Thunks going out of range.
+  //
+  // See comment in Arch/ARM.cpp for more detais of getThunkSectionSpacing().
+  return 0x8000 - 0x1000;
 }
 
 TargetInfo *elf::getPPC64TargetInfo() {


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