[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 --- Comment #6 from cqwrteur --- (In reply to Andrew Pinski from comment #5) > You already filed this one. > > *** This bug has been marked as a duplicate of bug 103550 *** (In reply to Andrew Pinski from comment #1) > This is a reassociation, scheduling issue and register allocation issue. > > Plus your example might be slower due to dependencies. > > Without a full example of where gcc ra goes wrong, gcc actually produces > much better code for this example due to register renaming in hw. > Note many x86_64 also does register renaming for the stack too On x86_64, I just checked uops.info, only two ports are available for rotr,rotl. They cannot really get paralleled executed.
[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 Andrew Pinski changed: What|Removed |Added Resolution|--- |DUPLICATE Status|UNCONFIRMED |RESOLVED --- Comment #5 from Andrew Pinski --- You already filed this one. *** This bug has been marked as a duplicate of bug 103550 ***
[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 --- Comment #4 from cqwrteur --- (In reply to Andrew Pinski from comment #1) > Plus your example might be slower due to dependencies. Dependency is only an issue to a certain degree. 1st one it has things like "movl%edi, %edx; rorl$11, %edx" which is also a flow dependency. CPU solves flow dependency to a very large degree with register forwarding. Write dependencies are also dealt with register renaming if we save registers, register renaming will also save time.
[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 --- Comment #3 from cqwrteur --- (In reply to Andrew Pinski from comment #1) > This is a reassociation, scheduling issue and register allocation issue. > > Plus your example might be slower due to dependencies. > > Without a full example of where gcc ra goes wrong, gcc actually produces > much better code for this example due to register renaming in hw. > Note many x86_64 also does register renaming for the stack too https://github.com/openssl/openssl/blob/a8572674f12ceb39f7e66ccbaa8918b922c76739/crypto/sha/asm/sha512-x86_64.pl#L16 They mentioned that before. 40% improvement over compiler-generated code. "I really wonder why gcc # [being armed with inline assembler] fails to generate as fast code." # sha256/512_block procedure for x86_64. # # 40% improvement over compiler-generated code on Opteron. On EM64T # sha256 was observed to run >80% faster and sha512 - >40%. No magical # tricks, just straight implementation... I really wonder why gcc # [being armed with inline assembler] fails to generate as fast code. # The only thing which is cool about this module is that it's very # same instruction sequence used for both SHA-256 and SHA-512. In # former case the instructions operate on 32-bit operands, while in # latter - on 64-bit ones. All I had to do is to get one flavor right, # the other one passed the test right away:-)
[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 --- Comment #2 from cqwrteur --- (In reply to Andrew Pinski from comment #1) > This is a reassociation, scheduling issue and register allocation issue. > > Plus your example might be slower due to dependencies. > > Without a full example of where gcc ra goes wrong, gcc actually produces > much better code for this example due to register renaming in hw. > Note many x86_64 also does register renaming for the stack too The problem I do things like sha512_round: sha512_round(x[0]=big_endian(W[0]),a,b,d,e,f,g,h,bpc,0x428a2f98d728ae22); sha512_round(x[1]=big_endian(W[1]),h,a,c,d,e,f,g,bpc,0x7137449123ef65cd); sha512_round(x[2]=big_endian(W[2]),g,h,b,c,d,e,f,bpc,0xb5c0fbcfec4d3b2f); sha512_round(x[3]=big_endian(W[3]),f,g,a,b,c,d,e,bpc,0xe9b5dba58189dbbc); sha512_round(x[4]=big_endian(W[4]),e,f,h,a,b,c,d,bpc,0x3956c25bf348b538); They use tons of registers. If GCC wastes registers, tons of time would waste on stack push/load. My implementation by GCC on x86_64 is slower than openssl's asm version particularly due to this reason. GCC just pushes/stores too many values on the stack. https://github.com/openssl/openssl/blob/master/crypto/sha/asm/sha512-x86_64.pl#L192 OpenSSL does exactly what I do here.
[Bug rtl-optimization/107167] It looks like GCC wastes registers on trivial computations when result can be cached
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 Andrew Pinski changed: What|Removed |Added Severity|normal |enhancement Keywords||missed-optimization --- Comment #1 from Andrew Pinski --- This is a reassociation, scheduling issue and register allocation issue. Plus your example might be slower due to dependencies. Without a full example of where gcc ra goes wrong, gcc actually produces much better code for this example due to register renaming in hw. Note many x86_64 also does register renaming for the stack too
