[lld] [lld][ELF] Fix a corner case of elf::getLoongArchPageDelta (PR #71907)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lld-elf

@llvm/pr-subscribers-lld

Author: Lu Weining (SixWeining)

<details>
<summary>Changes</summary>

If `page(dest) - page(pc)` is 0xfffffffffff000, i.e. page(pc) is next to page(dest), and lo12(dest) > 0x7ff, correct %pc64_lo12 and %pc64_hi12 should be both -1 (which can be checked with binutils) but they are both 0 on lld. This patch fixes this issue.

---
Full diff: https://github.com/llvm/llvm-project/pull/71907.diff


2 Files Affected:

- (modified) lld/ELF/Arch/LoongArch.cpp (+4) 
- (modified) lld/test/ELF/loongarch-pc-aligned.s (+5-6) 


``````````diff
diff --git a/lld/ELF/Arch/LoongArch.cpp b/lld/ELF/Arch/LoongArch.cpp
index 04ddb4682917b4b..01e6b037eb900af 100644
--- a/lld/ELF/Arch/LoongArch.cpp
+++ b/lld/ELF/Arch/LoongArch.cpp
@@ -159,6 +159,10 @@ uint64_t elf::getLoongArchPageDelta(uint64_t dest, uint64_t pc) {
   bool negativeA = lo12(dest) > 0x7ff;
   bool negativeB = (result & 0x8000'0000) != 0;
 
+  // A corner case; directly return the expected result.
+  if (result == 0xfffffffffffff000 && negativeA)
+    return result = 0xffffffff00000000;
+
   if (negativeA)
     result += 0x1000;
   if (negativeA && !negativeB)
diff --git a/lld/test/ELF/loongarch-pc-aligned.s b/lld/test/ELF/loongarch-pc-aligned.s
index f6ac56e5261ddb7..ae77376ebcdbf38 100644
--- a/lld/test/ELF/loongarch-pc-aligned.s
+++ b/lld/test/ELF/loongarch-pc-aligned.s
@@ -260,18 +260,17 @@
 # EXTREME15-NEXT: lu32i.d   $t0, -349526
 # EXTREME15-NEXT: lu52i.d   $t0, $t0, -1093
 
-## FIXME: Correct %pc64_lo20 should be 0xfffff (-1) and %pc64_hi12 should be 0xfff (-1), but current values are:
-## page delta = 0x0000000000000000, page offset = 0x888
+## page delta = 0xffffffff00000000, page offset = 0x888
 ## %pc_lo12   = 0x888 = -1912
 ## %pc_hi20   = 0x00000 = 0
-## %pc64_lo20 = 0x00000 = 0
-## %pc64_hi12 = 0x00000 = 0
+## %pc64_lo20 = 0xfffff = -1
+## %pc64_hi12 = 0xfff = -1
 # RUN: ld.lld %t/extreme.o --section-start=.rodata=0x0000000012344888 --section-start=.text=0x0000000012345678 -o %t/extreme16
 # RUN: llvm-objdump -d --no-show-raw-insn %t/extreme16 | FileCheck %s --check-prefix=EXTREME16
 # EXTREME16:      addi.d $t0, $zero, -1912
 # EXTREME16-NEXT: pcalau12i $t1, 0
-# EXTREME16-NEXT: lu32i.d   $t0, 0
-# EXTREME16-NEXT: lu52i.d   $t0, $t0, 0
+# EXTREME16-NEXT: lu32i.d   $t0, -1
+# EXTREME16-NEXT: lu52i.d   $t0, $t0, -1
 
 #--- a.s
 .rodata

``````````

</details>


https://github.com/llvm/llvm-project/pull/71907