fix #21435 Bad codegen when building dmd with optimizations

[rainers]

don't handle postinc/dec of *p++ again in cdeq(), it is already done in cdpost() and only modifies the temporary copy, sometimes even a different register.

[dlang-bot]

Thanks for your pull request, @rainers!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#21436"

[Herringway]

It doesn't look like the test is being built with optimizations?

[rainers]

It doesn't look like the test is being built with optimizations?

Tests should be built with all combinations of -g -O -inline -release, all but -g seem to be necessary to trigger the bad register usage.

[rainers]

Hi @WalterBright , is it actually ok to disable this code? It generates the gratuitious add after *p++ = 'A', e.g.:

0000000000000107: 48 8B D9           mov         rbx,rcx
000000000000010A: 48 FF C1           inc         rcx
000000000000010D: C6 03 41           mov         byte ptr [rbx],41h
0000000000000110: 48 83 C3 01        add         rbx,1

Is it used in some situation not covered by the test suite? If not, should the use of postinc be removed completely from cdeq?

I suspect the bug causing RSP to be modified by the addition is still lurking in idxregm, though: if the used register happens to be R12, this condition

dmd/compiler/src/dmd/backend/x86/cod2.d

Line 100 in 430661d

if ((rm & 7) == 4) /* if sib byte */

is true and idxregm returns a mask with bits 4 and 12 set, and findreg then selects 4 (RSP).