On Sun, Dec 5, 2010 at 8:56 AM, Chris Lattner <clatt...@apple.com> wrote: > > On Dec 4, 2010, at 5:22 AM, Florian Weimer wrote: > >> * Joe Buck: >> >>> It's wasted code if the multiply instruction detects the overflow. >>> It's true that the cost is small (maybe just one extra instruction >>> and the same number of tests, maybe one more on architectures where you >>> have to load a large constant), but it is slightly worse code than what >>> Chris Lattner showed. >> >> It's possible to improve slightly on the LLVM code by using the >> overflow flag (at least on i386/amd64), as explained in this blog >> post: >> >> <http://blogs.msdn.com/b/michael_howard/archive/2005/12/06/500629.aspx> > > Ah, great point. I improved the clang codegen to this: > > $ cat t.cc > void *test(long count) { > return new int[count]; > } > $ clang t.cc -S -o - -O3 -mkernel -fomit-frame-pointer -mllvm > -show-mc-encoding > .section __TEXT,__text,regular,pure_instructions > .globl __Z4testl > .align 4, 0x90 > __Z4testl: ## @_Z4testl > ## BB#0: ## %entry > movl $4, %ecx ## encoding: [0xb9,0x04,0x00,0x00,0x00] > movq %rdi, %rax ## encoding: [0x48,0x89,0xf8] > mulq %rcx ## encoding: [0x48,0xf7,0xe1] > movq $-1, %rdi ## encoding: > [0x48,0xc7,0xc7,0xff,0xff,0xff,0xff] > cmovnoq %rax, %rdi ## encoding: [0x48,0x0f,0x41,0xf8] > jmp __Znam ## TAILCALL > ## encoding: [0xeb,A] > ## fixup A - offset: 1, value: > __Znam-1, kind: FK_PCRel_1 > .subsections_via_symbols > > This could be further improved by inverting the cmov condition to avoid the > first movq, which we'll tackle as a general regalloc improvement.
I'm curious as on how you represent the overflow checking in your highlevel IL. Richard. > Thanks for the pointer! > > -Chris > >
