[TUHS] OT: compiler back-end bug

[Steve Johnson]

I have all to much experience with back end bugs, usually when someone
porting PCC asked for help.

The advice I always gave first was:  "what would the correct output
look like?"

90% of the time, they didn't know.  And it's hard to hit the target
if you don't know where it is...

Once you know what you want, then you figure out the first instruction
that isn't right and hit it with everything you have...

Hope this helps...

Steve

----- Original Message -----
From: "Ralph Corderoy" <ralph at inputplus.co.uk>
To:<tuhs at tuhs.org>
Cc:
Sent:Sun, 29 Sep 2019 11:50:16 +0100
Subject:Re: [TUHS] OT: compiler back-end bug

 Hi Warren,

 > Good point Ralph:
 >
https://minnie.tuhs.org/wktcloud/index.php/s/HQjsggHb4i6wdWM?path=%2FSfiles

 I've always tried to avoid x86 and friends for ARM, so I may be
wrong,
 but the run up to the first of the two memcpy() calls looks the same
to
 me. Here's the assembler, values given an RBP of 100, and the stack
 contents. Good version first, bad second.

 rbp = 100
 L29:
 movq -8(%rbp),%rax rax = *92
 pushq %rax *92
 movq 16(%rbp),%rax rax = *116
 pushq %rax *92 *116
 movq $64,%rax rax = 64
 pushq %rax *92 *116 64
 movq 32(%rbp),%rax rax = *132
 popq %rcx rcx = 64 *92 *116
 addq %rcx,%rax rcx = 64+*132
 movq (%rax),%rax rax = *(64+*132)
 pushq %rax *92 *116 *(64+*132)
 movq $40,%rax rax = 40
 pushq %rax *92 *116 *(64+*132) 40
 movq 32(%rbp),%rax rax = *132
 popq %rcx rcx = 40 *92 *116 *(64+*132)
 addq %rcx,%rax rax = 40+*132
 movq (%rax),%rax rax = *(40+*132)
 popq %rcx rcx = *(64+*132) *92 *116
 addq %rcx,%rax rax = *(64+*132)+*(40+*132)
 pushq %rax *92 *116 *(64+*132)+*(40+*132)
 call Cmemcpy

 rbp = 100
 L29:
 movq -8(%rbp),%r8 r8 = *92
 pushq %r8 *92
 movq 16(%rbp),%r8 r8 = *116
 pushq %r8 *92 *116
 movq $64,%r8 r8 = 64
 movq 32(%rbp),%r9 r9 = *132
 addq %r9,%r8 r8 = *132+64
 movq (%r8),%r8 r8 = *(*132+64)
 movq $40,%r9 r9 = 40
 movq 32(%rbp),%r10 r10 = *132
 addq %r10,%r9 r9 = *132+40
 movq (%r9),%r9 r9 = *(*132+40)
 addq %r9,%r8 r8 = *(*132+64)+*(*132+40)
 pushq %r8 *92 *116 *(*132+64)+*(*132+40)
 call Cmemcpy

 A glance at the second memcpy() call look equivalent too.

 So perhaps it's not calculating the parameters to memcpy() that's
wrong,
 but the inputs into those calculations being faulty? I'd use gdb(1)
to
 break at particular instructions, examine memory, etc., to work
 backwards through the bad version until spotting where good data
becomes
 bad.

 -- 
 Cheers, Ralph.


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