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Created attachment 1200444 [details] Example code Description of problem: In certain cases gcc wants to generate the equivalent of move.b (%a0,-1),foo but instead of generating moveq #-1.%d0, moveb(%a0,%d0.l) or similar it generates the bogus sequence moveq #0,%d0 not.w %d0 (the fast way of generating a word value -1 not a long) and as a result instead randomly pees on memory 64K offset Version-Release number of selected component (if applicable): 5.3.1 2.fc23 x86_64 How reproducible: 100% Steps to Reproduce: 1. Compile the attached sample c code with gcc -mshort -m68000 -O2 -S 1.c 2. Examine 1.s 3 #facepalm Actual results: Complete nonsense (bogus noveq/not, bogus ext.l and leal.l of undefined %d0 value higher up) Expected results: A memcpy function Additional info: Building with -Os produces very different but valid code, building without -mshort produces valid code so it looks like -O2 -mshort triggers a bogus optimisation
(In reply to alan from comment #0) > Complete nonsense (bogus noveq/not, bogus ext.l and leal.l of undefined %d0 > value higher up) You didn't paste the asm you got, but in the asm I get from 6.1.1, %d0 is validly loaded with the size parameter: link.w %fp,#0 move.l %a3,-(%sp) move.l %a2,-(%sp) move.l 8(%fp),%a0 move.l 16(%fp),%d0 Looking at the frame layout: fp+ 0 -> prev frame pointer fp+ 4 -> ret addr fp+ 8 -> param d fp+12 -> param s fp+16 -> param sz on the other hand, passing it through ext.l would seem to be wrong, given that it's a long int, not an int. With regard to the main point of this BZ, rather than generating: moveb(%a0,%d0.l) it should perhaps be generating: moveb(%a0,%d0.w) and then having 0xffff in %d0 isn't a problem as it's sign-extended.
5.x generates link.w %fp,#0 move.l %a3,-(%sp) move.l %a2,-(%sp) move.l 8(%fp),%a0 move.l 12(%fp),%a1 move.l 16(%fp),%d0 jeq .L6 move.l %a0,%a2 ext.l %d0 lea (%a0,%d0.l),%a3 .L3: addq.l #1,%a1 moveq #0,%d0 <----- not.w %d0 <----- Should be moveq #-1,%d0 ? move.b (%a1,%d0.l),(%a2)+ <----- Or just addq to a1 *after* the load ! cmp.l %a2,%a3 jne .L3 .L6: move.l %a0,%d0 move.l (%sp)+,%a2 move.l (%sp)+,%a3 unlk %fp rts
To quote Jeff Law on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77600#c2 : This is a similar issue as I outlined in the other m68k -mshort BZ. This time we need to look at how INTPTR_TYPE, which is defined in terms of LONG_TYPE_SIZE. When LONG_TYPE_SIZE == 32, INTPTR_TYPE will be an "int" -mshort changes the size of an int from 32 to 16 bits. Thus conversions to/from sizetype are going to do things you don't expect unless you know sizetype is 16 bits. Again, if you configure for a bare metal target such as m68k-elf or avoid using -mshort on your linux target, you'll get the desired and expected result.
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