Wrong Optimization on -O2 -O3 of register shifts

Summary

Some code was wrongly optimized, resulting in incorrect generated assembly.

System Information

Tested on Linux x86_64 with FPC trunk. Run with -O3 or -O2 would fail. Run with -O1 or -O0 would pass.

Steps to reproduce

Compile and run the following code snippet:

function strspn(s, accept: pointer): integer;
var
  p: PCardinal;
  c: AnsiChar;
  d: cardinal;
begin
  // returns size of initial segment of s which are in accept
  result := 0;
  repeat
    c := PAnsiChar(s)[result];
    if c = #0 then
      break;
    p := accept;
    repeat // stop as soon as we find any character not from accept
      d := p^;
      inc(p);
      if AnsiChar(d) = c then
        break
      else if AnsiChar(d) = #0 then
        exit;
      d := d shr 8;
      if AnsiChar(d) = c then
        break
      else if AnsiChar(d) = #0 then
        exit;
      d := d shr 8;
      if AnsiChar(d) = c then
        break
      else if AnsiChar(d) = #0 then
        exit;
      d := d shr 8;
      if AnsiChar(d) = c then
        break
      else if AnsiChar(d) = #0 then
        exit;
    until false;
    inc(result);
  until false;
end;

procedure TestIt;
begin
  writeln(strspn(PAnsiChar('abcdef'), PAnsiChar('debca')));
  // it should return 5 - but 1 is returned
end;

What is the current bug behavior?

It displays 1 with -O3 or -O2. Note that run with -O1 or -O0 would pass and return 5 as expected.

What is the expected (correct) behavior?

It should display the number 5. As it was the case with FPC 3.2.2 or with -O1 or -O0.

Relevant logs and/or screenshots

Here is the generated asm, which is clearly wrong:

/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6221  result := 0;
00000000004D2E60 31C0                     xor eax,eax
00000000004D2E62 660F1F440000             nop word ptr [rax+rax+$00]
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6223  c := PAnsiChar(s)[result];
00000000004D2E68 4863D0                   movsxd rdx,eax
00000000004D2E6B 448A0C17                 mov r9b,[rdx+rdi]
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6224  if c = #0 then
00000000004D2E6F 4584C9                   test r9b,r9b
00000000004D2E72 7445                     jz +$45    # $00000000004D2EB9 strspn+89 mormot.core.unicode.pas:6252
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6226  p := accept;
00000000004D2E74 4889F1                   mov rcx,rsi
00000000004D2E77 660F1F840000000000       nop word ptr [rax+rax+$00000000]
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6228  d := p^;
00000000004D2E80 448B01                   mov r8d,[rcx]
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6229  inc(p);
00000000004D2E83 4883C104                 add rcx,$04
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6230  if AnsiChar(d) = c then
00000000004D2E87 4538C8                   cmp r8b,r9b
00000000004D2E8A 7428                     jz +$28    # $00000000004D2EB4 strspn+84 mormot.core.unicode.pas:6250
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6232  else if AnsiChar(d) = #0 then
00000000004D2E8C 4584C0                   test r8b,r8b
00000000004D2E8F 7428                     jz +$28    # $00000000004D2EB9 strspn+89 mormot.core.unicode.pas:6252
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6234  d := d shr 8;
00000000004D2E91 41C1E818                 shr r8d,$18
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6235  if AnsiChar(d) = c then
00000000004D2E95 4538C8                   cmp r8b,r9b
00000000004D2E98 741A                     jz +$1A    # $00000000004D2EB4 strspn+84 mormot.core.unicode.pas:6250
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6237  else if AnsiChar(d) = #0 then
00000000004D2E9A 4584C0                   test r8b,r8b
00000000004D2E9D 741A                     jz +$1A    # $00000000004D2EB9 strspn+89 mormot.core.unicode.pas:6252
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6240  if AnsiChar(d) = c then
00000000004D2E9F 4538C8                   cmp r8b,r9b
00000000004D2EA2 7410                     jz +$10    # $00000000004D2EB4 strspn+84 mormot.core.unicode.pas:6250
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6242  else if AnsiChar(d) = #0 then
00000000004D2EA4 4584C0                   test r8b,r8b
00000000004D2EA7 7410                     jz +$10    # $00000000004D2EB9 strspn+89 mormot.core.unicode.pas:6252
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6245  if AnsiChar(d) = c then
00000000004D2EA9 4538C8                   cmp r8b,r9b
00000000004D2EAC 7406                     jz +$06    # $00000000004D2EB4 strspn+84 mormot.core.unicode.pas:6250
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6247  else if AnsiChar(d) = #0 then
00000000004D2EAE 4584C0                   test r8b,r8b
00000000004D2EB1 75CD                     jnz -$33    # $00000000004D2E80 strspn+32 mormot.core.unicode.pas:6228
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6249  until false;
00000000004D2EB3 C3                       ret 
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6250  inc(result);
00000000004D2EB4 83C001                   add eax,$01
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6251  until false;
00000000004D2EB7 EBAF                     jmp -$51    # $00000000004D2E68 strspn+8 mormot.core.unicode.pas:6223
/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6252  end;
00000000004D2EB9 C3                       ret 

The code is reading the input per 32-bit values at once, then try the last 8-bit, making a shr 8 between each try. The several d := d shr 8 were wrongly optimized to:

/home/ab/github/mORMot2/src/core/mormot.core.unicode.pas:6234  d := d shr 8;
00000000004D2E91 41C1E818                 shr r8d,$18