[Mips] AttemptToFoldSymbolOffsetDifference: revert isMicroMips special case

[MaskRay]

Summary

D52985/D57677 added a .gcc_except_table workaround, but the new behavior
doesn't match GNU assembler.

void foo();
int bar() {
  foo();
  try { throw 1; }
  catch (int) { return 1; }
  return 0;
}

clang --target=mipsel-linux-gnu -mmicromips -S a.cc
mipsel-linux-gnu-gcc -mmicromips -c a.s -o gnu.o

.uleb128 ($cst_end0)-($cst_begin0)     // bit 0 is not forced to 1
.uleb128 ($func_begin0)-($func_begin0) // bit 0 is not forced to 1

I have inspected .gcc_except_table output by mipsel-linux-gnu-gcc -mmicromips -c a.cc.
The .uleb128 values are not forced to set the least significant bit.

In addition, D57677's adjustment (even->odd) to CodeGen/Mips/micromips-b-range.ll is wrong.
PC-relative .long func - . values will differ from GNU assembler as well.

The original intention of D52985 seems unclear to me. I think whatever
goal it wants to achieve should be moved to an upper layer.

This isMicroMips special case has caused problems to fix MCAssembler::relaxLEB to use evaluateAsAbsolute instead of evaluateKnownAbsolute,
which is needed to proper support R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128.

[MaskRay]

llvm/test/CodeGen/Mips/micromips-b-range.ll

16

I left a comment to https://reviews.llvm.org/D57677#4594051 . I am now more certain that the micromips .gcc_except_table is wrong and harmful and I am going to land this revert.

Nobody has noticed an issue likely because micromips support is quite poor in llvm.

[MaskRay]

describe better why D52985/D57677 are wrong and should be reverted

llvm/lib/MC/MCExpr.cpp

if (!Asm->getWriter().isSymbolRefDifferenceFullyResolved(*Asm, A, B, InSet))
     return;
 
   auto FinalizeFolding = [&]() {
     // Pointers to Thumb symbols need to have their low-bit set to allow
     // for interworking.
     if (Asm->isThumbFunc(&SA))
       Addend |= 1;
 
-    // If symbol is labeled as micromips, we set low-bit to ensure
-    // correct offset in .gcc_except_table
-    if (Asm->getBackend().isMicroMips(&SA))
-      Addend |= 1;
-
     // Clear the symbol expr pointers to indicate we have folded these
     // operands.
     A = B = nullptr;
   };
 
   const MCFragment *FA = SA.getFragment();
   const MCFragment *FB = SB.getFragment();
   const MCSection &SecA = *FA->getParent();

llvm/test/CodeGen/Mips/micromips-b-range.ll

 ; RUN: llc -march=mips -relocation-model=pic -mattr=+micromips \
 ; RUN:     -filetype=obj -o - %s | llvm-objdump --no-print-imm-hex -d - | FileCheck %s
 
 ; CHECK-LABEL: <foo>:
 ; CHECK-NEXT:     0:  41 a2 00 00   lui $2, 0
 ; CHECK-NEXT:     4:  30 42 00 00   addiu $2, $2, 0
 ; CHECK-NEXT:     8:  03 22 11 50   addu  $2, $2, $25
 ; CHECK-NEXT:     c:  fc 42 00 00   lw  $2, 0($2)
 ; CHECK-NEXT:    10:  69 20         lw16  $2, 0($2)
 ; CHECK-NEXT:    12:  40 c2 00 14   bgtz  $2, 0x3e <foo+0x3e>
 ; CHECK-NEXT:    16:  00 00 00 00   nop
 ; CHECK-NEXT:    1a:  33 bd ff f8   addiu $sp, $sp, -8
 ; CHECK-NEXT:    1e:  fb fd 00 00   sw  $ra, 0($sp)
 ; CHECK-NEXT:    22:  41 a1 00 01   lui $1, 1
 ; CHECK-NEXT:    26:  40 60 00 02   bal 0x2e <foo+0x2e>
-; CHECK-NEXT:    2a:  30 21 04 69   addiu $1, $1, 1129
+; CHECK-NEXT:    2a:  30 21 04 68   addiu $1, $1, 1128
 ; CHECK-NEXT:    2e:  00 3f 09 50   addu  $1, $ra, $1
 ; CHECK-NEXT:    32:  ff fd 00 00   lw  $ra, 0($sp)
 ; CHECK-NEXT:    36:  00 01 0f 3c   jr  $1
 ; CHECK-NEXT:    3a:  33 bd 00 08   addiu $sp, $sp, 8
 ; CHECK-NEXT:    3e:  94 00 00 02   b 0x46 <foo+0x46>
 ; CHECK-NEXT:    42:  00 00 00 00   nop
 ; CHECK-NEXT:    46:  30 20 4e 1f   addiu $1, $zero, 19999
 ; CHECK-NEXT:    4a:  b4 22 00 14   bne $2, $1, 0x76 <foo+0x76>
 ; CHECK-NEXT:    4e:  00 00 00 00   nop
 ; CHECK-NEXT:    52:  33 bd ff f8   addiu $sp, $sp, -8
 ; CHECK-NEXT:    56:  fb fd 00 00   sw  $ra, 0($sp)
 ; CHECK-NEXT:    5a:  41 a1 00 01   lui $1, 1
 ; CHECK-NEXT:    5e:  40 60 00 02   bal 0x66 <foo+0x66>
-; CHECK-NEXT:    62:  30 21 04 5d   addiu $1, $1, 1117
+; CHECK-NEXT:    62:  30 21 04 5c   addiu $1, $1, 1116
 ; CHECK-NEXT:    66:  00 3f 09 50   addu  $1, $ra, $1
 ; CHECK-NEXT:    6a:  ff fd 00 00   lw  $ra, 0($sp)
 ; CHECK-NEXT:    6e:  00 01 0f 3c   jr  $1
 ; CHECK-NEXT:    72:  33 bd 00 08   addiu $sp, $sp, 8
 ; CHECK-NEXT:    76:  30 20 27 0f   addiu $1, $zero, 9999
 ; CHECK-NEXT:    7a:  94 22 00 14   beq $2, $1, 0xa6 <foo+0xa6>
 ; CHECK-NEXT:    7e:  00 00 00 00   nop
 ; CHECK-NEXT:    82:  33 bd ff f8   addiu $sp, $sp, -8
 ; CHECK-NEXT:    86:  fb fd 00 00   sw  $ra, 0($sp)
 ; CHECK-NEXT:    8a:  41 a1 00 01   lui $1, 1
 ; CHECK-NEXT:    8e:  40 60 00 02   bal 0x96 <foo+0x96>
-; CHECK-NEXT:    92:  30 21 04 2d   addiu $1, $1, 1069
+; CHECK-NEXT:    92:  30 21 04 2c   addiu $1, $1, 1068
 ; CHECK-NEXT:    96:  00 3f 09 50   addu  $1, $ra, $1
 ; CHECK-NEXT:    9a:  ff fd 00 00   lw  $ra, 0($sp)
 ; CHECK-NEXT:    9e:  00 01 0f 3c   jr  $1
 ; CHECK-NEXT:    a2:  33 bd 00 08   addiu $sp, $sp, 8
 ; CHECK:                ...
 ; CHECK-NEXT: 1046a:  94 00 00 02   b 0x10472 <foo+0x10472>
 ; CHECK-NEXT: 1046e:  00 00 00 00   nop
 ; CHECK-NEXT: 10472:  33 bd ff f8   addiu $sp, $sp, -8
 ; CHECK-NEXT: 10476:  fb fd 00 00   sw  $ra, 0($sp)
 ; CHECK-NEXT: 1047a:  41 a1 00 01   lui $1, 1
 ; CHECK-NEXT: 1047e:  40 60 00 02   bal 0x10486 <foo+0x10486>
-; CHECK-NEXT: 10482:  30 21 04 01   addiu $1, $1, 1025
+; CHECK-NEXT: 10482:  30 21 04 00   addiu $1, $1, 1024
 ; CHECK-NEXT: 10486:  00 3f 09 50   addu  $1, $ra, $1
 ; CHECK-NEXT: 1048a:  ff fd 00 00   lw  $ra, 0($sp)
 ; CHECK-NEXT: 1048e:  00 01 0f 3c   jr  $1
 ; CHECK-NEXT: 10492:  33 bd 00 08   addiu $sp, $sp, 8
 ; CHECK-NEXT: 10496:  94 00 00 02   b 0x1049e <foo+0x1049e>
 
 @x = external global i32, align 4
 

llvm/test/CodeGen/Mips/micromips-gcc-except-table.ll

 ; RUN: llc -mtriple=mips-linux-gnu -mcpu=mips32r2 -mattr=+micromips -O3 -filetype=obj < %s | llvm-objdump -s -j .gcc_except_table - | FileCheck %s
 
 ; CHECK: Contents of section .gcc_except_table:
-; CHECK-NEXT: 0000 ff9b1501 0c011100 00110e1f 011f1800
+; CHECK-NEXT: 0000 ff9b1501 0c001000 00100e1e 011e1800
 ; CHECK-NEXT: 0010 00010000 00000000
 
 @_ZTIi = external constant ptr
 
 define dso_local i32 @main() local_unnamed_addr norecurse personality ptr @__gxx_personality_v0 {
 entry:
   %exception.i = tail call ptr @__cxa_allocate_exception(i32 4) nounwind
   store i32 5, ptr %exception.i, align 16

llvm/test/DebugInfo/Mips/eh_frame.ll

 ; PIC-DAG: R_MIPS_PC32
 ; PIC-DAG: R_MIPS_32   00000000 .gcc_except_table
 
 ; CHECK-READELF: DW.ref.__gxx_personality_v0
 ; CHECK-READELF-STATIC-NEXT: R_MIPS_32 00000000 .text
 ; CHECK-READELF-PIC-NEXT: R_MIPS_PC32
 ; CHECK-READELF-NEXT: .gcc_except_table
 
-; EXCEPT-TABLE-STATIC: 0000 ff9b1501 0c011500 00150e23 01231e00  ...........#.#..
+; EXCEPT-TABLE-STATIC: 0000 ff9b1501 0c001400 00140e22 01221e00 ..........."."..
 ; EXCEPT-TABLE-STATIC: 0010 00010000 00000000
-; EXCEPT-TABLE-PIC:    0000 ff9b1501 0c012d00 002d133f 013f2a00 ......-..-.?.?*.
+; EXCEPT-TABLE-PIC:    0000 ff9b1501 0c002c00 002c123e 013e2a00 ......,..,.>.>*.
 ; EXCEPT-TABLE-PIC:    0010 00010000 00000000                    ........
 
 @_ZTIi = external constant ptr
 
 define dso_local i32 @main() local_unnamed_addr personality ptr @__gxx_personality_v0 {
 entry:
   %exception.i = tail call ptr @__cxa_allocate_exception(i32 4) nounwind
   store i32 5, ptr %exception.i, align 16