Bug 20441 – Wrong code with -O -fPIC and pointer subtraction

This reduced test case tested with 2.086, but original bug was found with 2.089 and 2.078. Assert fails if compiled with -O -fPIC. Passes if compiled without -O. --- const(char)* moo(const (char) *s) { return s; } void main () { const(char) *x = "abc".ptr; assert( moo(x) - x == 0 ); } ---

Can reproduce on OSX with `dmd -O -run ...`

On any non-ancient DMD version, -fPIC is enabled by default in the dmd.conf file. I have run the testcase on 2.078 with -fPIC removed from the dmd.conf file. It requires both -fPIC and -O to trigger the bug.

And here is what the optimizer produces. The problematic instruction is highlighted. 0x000055555558dbe8 <+0>: push %rbp 0x000055555558dbe9 <+1>: mov %rsp,%rbp 0x000055555558dbec <+4>: lea 0x31d6d(%rip),%rdi # 0x5555555bf960 0x000055555558dbf3 <+11>: callq 0x55555558dbdc <_moo> => 0x000055555558dbf8 <+16>: sub $0x31d66,%rax 0x000055555558dbfe <+22>: jne 0x55555558dc04 <_Dmain+28> 0x000055555558dc00 <+24>: xor %eax,%eax 0x000055555558dc02 <+26>: pop %rbp 0x000055555558dc03 <+27>: retq The mistake is that the addressing mode flag to make it RIP-relative, is missing. Instead of sub RAX, [RIP + 0x31d66] it is generating sub RAX, [0x31d66]

Wrong code is generated for any of the 'orthogonal' operators. This is another example. size_t roo(size_t r) { return r; } void main () { size_t y = cast(size_t)("abc".ptr); assert ( (roo(y)^y) == 0); } Bug probably exists in cdorth() in cod2.d. As far as I can tell, that function never generates RIP-relative addressing under any circumstances.

@WalterBright created dlang/dmd pull request #10661 "fix Issue 20441 - Wrong code with -O -fPIC and pointer subtraction" fixing this issue: - fix Issue 20441 - Wrong code with -O -fPIC and pointer subtraction https://github.com/dlang/dmd/pull/10661

dlang/dmd pull request #10661 "fix Issue 20441 - Wrong code with -O -fPIC and pointer subtraction" was merged into stable: - a4bc86ce3a0c4122fe27c84ae801c42f1c8edeef by Walter Bright: fix Issue 20441 - Wrong code with -O -fPIC and pointer subtraction https://github.com/dlang/dmd/pull/10661