On August 12, 2015 7:02:04 PM GMT+02:00, Jeff Law <l...@redhat.com> wrote: >On 08/12/2015 07:33 AM, Mikael Morin wrote: >> Le 12/08/2015 13:09, Richard Biener a écrit : >>> On Wed, Aug 12, 2015 at 1:07 PM, Markus Trippelsdorf >>> <mar...@trippelsdorf.de> wrote: >>>> On 2015.08.12 at 13:01 +0200, Richard Biener wrote: >>>>> What kind of error does ubsan run into? That is, for which >'prec'? >>>> >>>> See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67042 >>> >>> Ugh. Stupid C. >>> >>> My fear is that with so many stmts sext_hwi isn't any longer >>> considered for >>> inlining, even if prec is constant. What's the generated code for >its >>> out-of line >>> copy with/without the patch? >>> >> >> The difference on x86_64 is: >> >> .cfi_startproc >> cmpl $64, %esi >> je .L3 >> - movl $64, %ecx >> - subl %esi, %ecx >> - salq %cl, %rdi >> + leal -1(%rsi), %ecx >> + movl $1, %eax >> + movq %rax, %rdx >> + salq %cl, %rdx >> + movl %esi, %ecx >> + salq %cl, %rax >> + subq $1, %rax >> + andq %rax, %rdi >> + xorq %rdx, %rdi >> movq %rdi, %rax >> - sarq %cl, %rax >> + subq %rdx, %rax >> ret >> .p2align 4,,10 >> .p2align 3 >> >> with -O2, tests attached. >> Yes it's worse. I thought it was better to avoid undefined behaviour >at >> all prices to avoid "bad surprises". >Well, this isn't the right test. You're testing when PREC is an >unknown >and I fully expect in that case the code you're going to get will be >worse. There's too many insns for combine to save us in that case. > >What's interesting here is what happens when prec is a known value >(since we're hoping that's the common case via inlining). > >When prec has a known value of 8, 16 or 32 we get a nice movs[bwl]q on >x86_64, which is exactly what we want. > >When prec is another known value, say 13 for the sake of argument, we >get a pair of shifts, which is again exactly what we want. > >So when prec is a constant, we're going to get good code. So the only >question left is whether or not prec is usually a constant or not in >the >contexts in which this routine is used.
Prec is almost never a constant and is heavily used from wide-int. We are not exploiting this undefined ness in C so I object to making this so much slower. Can we instead do what we do for abs_hwi and add a checking assert so we can move the tests to the callers that misbehave instead? Thanks, Richard. > >Jeff
