## Summary The optimizer produces bad asm code in the user app. Using registers for local vars, the peephole swaps some registers, but later uses the original register which then is not initialized. ## System Information - **Operating system:** Windows 10 / 64 bit - **Processor architecture:** x86, Intel 64 bit - **Compiler version:** 3.3.1 d3ac07ad274b6c6b09ff9b08a9d7007aaf780aa6 ## Steps to reproduce Compile the example.<br/> Look at the asm code for `procedure TLazFixedRoundBufferListMemBase.MoveBytesToOther(AFromByteOffset, AToByteOffset, AByteCount, AByteCap: Integer; AnOther: PLazFixedRoundBufferListMemBase);` Must be `-O3` or `-O4` ### Register `r10` is used (value read from `r10`) without ever having been assigned `r10` does not get used/initialized in that code, but in ``` # Var OtherMPtr located in register r15 # Var PTarget located in register rdx # Register r10 released # [1728] DstHigh := PTarget + AByteCap; cltq leaq 16(%rax,%r10),%rdi ``` It is used as value for `OtherMPtr`. - `rax` is AByteCap` - The line is merged with `PTarget := @OtherMPtr^.Data;` and Data is at `@(OtherMPtr+16)^ ---- #### Comparing asm (snippets) with/without peephole ##### Block 1 - At the start (getting the value for `OtherMPtr`) * **Good** code with peephole **off** ``` # Register r9 allocated # [1720] OtherMPtr := AnOther^.FMem.FMem; movq 56(%rbp),%r9 # Var AnOther located in register r9 # Var OtherMPtr located in register r10 # Register r10 allocated movq (%r9),%r10 # Register r9 released ``` * **Bad** code with peephole **on** ``` # [1720] OtherMPtr := AnOther^.FMem.FMem; movq 56(%rbp),%r9 # Var AnOther located in register r9 # Var OtherMPtr located in register r10 # Register r10,r15,r15 allocated movq (%r9),%r15 # Register r9 released ``` - Note: - - With peephole `OtherMPtr` will be in r15 - - `Var OtherMPtr located in register r10` **is wrong** - - `# Register r10,r15,r15 allocated` **r10 is still marked allocated / r15 is marked allocated twice** ##### Block 2 - **The code that is wrong above** Below is the code **without peephole**. This is the correct version of the first asm snippet (with the wrong non initialized `r10` usage).<br/> - Here `r10` has the value of `OtherMPtr` - PTarget is correctly loaded (the asm for that line is not merged with the next line) - the faulty line then uses PTarget in rdx ``` # Register r15 allocated # [1727] PTarget := @OtherMPtr^.Data; movq %r10,%r15 # Register r10 released # Var OtherMPtr located in register r15 # Register rdx allocated leaq 16(%r15),%rdx # Var PTarget located in register rdx # [1728] DstHigh := PTarget + AByteCap; movslq %eax,%rax leaq (%rax,%rdx),%rax ``` Note that without the peephole, `OtherMPtr` is in `r10`. But gets moved to `r15` before `PTarget` is read. - With the peephole, `OtherMPtr` will go straight into `r15` in "block 1" (so that is good, saves the register move) - But the peephole then apparently (**I guess**) gets it wrong when combining the asm from the 2 lines. - - Instead of using `r15` it uses `r10` ignoring that it already changed that register. ## Example Project Attached [crash_opt_reg.zip](/uploads/c643e8e9db982a003ea6cb9a66f34686/crash_opt_reg.zip) - Compile with `fpc.exe -O3 -alr project1.lpr` - Enable `{$Optimization noPEEPHOLE}` in LazListClasses to get working code