[lld] [lld][LoongArch] Partially fix the handling of R_LARCH_PCALA64_* relocs (PR #73387)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld

Author: Lu Weining (SixWeining)

<details>
<summary>Changes</summary>

Defer the compution of `negativeB` because adding 0x1000 to original `result` may yield a different `negativeB` value. Actually this issue was first reported by @<!-- -->rui314 at https://reviews.llvm.org/D138135#<!-- -->4568594.

Note that even with this patch, the handling of R_LARCH_PCALA64_* relocs are NOT totally correct, because current approach assumes those four instructions (pcalau12i/addi.d/lu32i.d/lu52i.d) are in the same 4K-page which is not always true. It is possible to document this assumption as a constraint in psABI in future. But at least this patch is necessary.

See https://github.com/llvm/llvm-project/pull/71907 and https://github.com/loongson-community/discussions/issues/17 for details.

---
Full diff: https://github.com/llvm/llvm-project/pull/73387.diff


2 Files Affected:

- (modified) lld/ELF/Arch/LoongArch.cpp (+1-2) 
- (modified) lld/test/ELF/loongarch-pc-aligned.s (+10-12) 


``````````diff
diff --git a/lld/ELF/Arch/LoongArch.cpp b/lld/ELF/Arch/LoongArch.cpp
index 1c3e015efc1649a..9291ff14d674b64 100644
--- a/lld/ELF/Arch/LoongArch.cpp
+++ b/lld/ELF/Arch/LoongArch.cpp
@@ -157,10 +157,9 @@ uint64_t elf::getLoongArchPageDelta(uint64_t dest, uint64_t pc) {
   //     result = page(dest) - page(pc) + 0x1000
   uint64_t result = getLoongArchPage(dest) - getLoongArchPage(pc);
   bool negativeA = lo12(dest) > 0x7ff;
-  bool negativeB = (result & 0x8000'0000) != 0;
-
   if (negativeA)
     result += 0x1000;
+  bool negativeB = (result & 0x8000'0000) != 0;
   if (negativeA && !negativeB)
     result -= 0x10000'0000;
   else if (!negativeA && negativeB)
diff --git a/lld/test/ELF/loongarch-pc-aligned.s b/lld/test/ELF/loongarch-pc-aligned.s
index e7950400a5c8c45..742ac502e799804 100644
--- a/lld/test/ELF/loongarch-pc-aligned.s
+++ b/lld/test/ELF/loongarch-pc-aligned.s
@@ -260,31 +260,29 @@
 # 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
 
-## FIXME: Correct %pc64_lo20 should be 0x00000 (0) and %pc64_hi12 should be 0x000 (0), but current values are:
-## page delta = 0xffffffff80000000, page offset = 0x888
+## page delta = 0x0000000080000000, page offset = 0x888
 ## %pc_lo12   = 0x888 = -1912
 ## %pc_hi20   = 0x80000 = -524288
-## %pc64_lo20 = 0xfffff = -1
-## %pc64_hi12 = 0xfff = -1
+## %pc64_lo20 = 0xfffff = 0
+## %pc64_hi12 = 0xfff = 0
 # RUN: ld.lld %t/extreme.o --section-start=.rodata=0x000071238ffff888 --section-start=.text=0x0000712310000678 -o %t/extreme17
 # RUN: llvm-objdump -d --no-show-raw-insn %t/extreme17 | FileCheck %s --check-prefix=EXTREME17
 # EXTREME17:      addi.d $t0, $zero, -1912
 # EXTREME17-NEXT: pcalau12i $t1, -524288
-# EXTREME17-NEXT: lu32i.d   $t0, -1
-# EXTREME17-NEXT: lu52i.d   $t0, $t0, -1
+# EXTREME17-NEXT: lu32i.d   $t0, 0
+# EXTREME17-NEXT: lu52i.d   $t0, $t0, 0
 
 #--- a.s
 .rodata

``````````

</details>


https://github.com/llvm/llvm-project/pull/73387