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

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

adalava updated this revision to Diff 198688.
adalava added a comment.

update testcase to use readelf instead of objdump, so it doesn't depend on D61647 <https://reviews.llvm.org/D61647> anymore


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
===================================================================
--- /dev/null
+++ lld/test/ELF/ppc64-branch-thunkspacing.s
@@ -0,0 +1,30 @@
+# REQUIRES: ppc
+
+# 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-readelf --symbols %t2 | FileCheck %s
+
+.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
+	bl  too_far1
+
+
+# When ThunkSpacing is applied, "__long_branch_too_far1" is shifted
+#   to an offset after "_start"
+
+# CHECK: Symbol table '.symtab' contains 4 entries:
+# CHECK:      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
+# CHECK-NEXT:      1: 0000000010010018    16 FUNC    LOCAL  DEFAULT    2 __long_branch_too_far1
+# CHECK-NEXT:      2: 0000000010010000     0 FUNC    GLOBAL DEFAULT    2 _start
+# CHECK-NEXT:      3: 0000000014010000     0 FUNC    GLOBAL DEFAULT  ABS too_far1
+
Index: lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
===================================================================
--- /dev/null
+++ lld/test/ELF/Inputs/ppc64-far-branch-rangethunk.s
@@ -0,0 +1,9 @@
+
+.section .init,"ax", at progbits,unique,2
+.align 2
+.global too_far1
+.type too_far1,%function
+
+// place too_far1 to + 64MB (should be above 32MB)
+too_far1 = 0x10010000 + 0x4000000
+
Index: lld/ELF/Arch/PPC64.cpp
===================================================================
--- lld/ELF/Arch/PPC64.cpp
+++ lld/ELF/Arch/PPC64.cpp
@@ -211,6 +211,7 @@
 
   bool adjustPrologueForCrossSplitStack(uint8_t *Loc, uint8_t *End,
                                         uint8_t StOther) const override;
+  uint32_t getThunkSectionSpacing() const override;
 };
 } // namespace
 
@@ -1051,6 +1052,25 @@
   return true;
 }
 
+uint32_t PPC64::getThunkSectionSpacing() const {
+  // 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 less 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 0x80000 allow creating 32,768 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 0x2000000 - 0x80000;
+}
+
 TargetInfo *elf::getPPC64TargetInfo() {
   static PPC64 Target;
   return &Target;


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