From ba309d43aae75d155a71de91c10afc9486bbbefa Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Wed, 27 Feb 2008 01:17:20 +0000 Subject: [PATCH] add a note git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47652 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Target/X86/README-X86-64.txt | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/lib/Target/X86/README-X86-64.txt b/lib/Target/X86/README-X86-64.txt index bc19986eba4..bdff56d40bf 100644 --- a/lib/Target/X86/README-X86-64.txt +++ b/lib/Target/X86/README-X86-64.txt @@ -234,3 +234,30 @@ down by 8 and truncate it. It's not pretty but it works. We need some register allocation magic to make the hack go away (e.g. putting additional constraints on the result of the movb). +//===---------------------------------------------------------------------===// + +This function: +double a(double b) {return (unsigned)b;} +compiles to this code: + +_a: + subq $8, %rsp + cvttsd2siq %xmm0, %rax + movl $4294967295, %ecx + andq %rcx, %rax + cvtsi2sdq %rax, %xmm0 + addq $8, %rsp + ret + +note the dead rsp adjustments. Also, there is surely a better/shorter way +to clear the top 32-bits of a 64-bit register than movl+andq. Testcase here: + +unsigned long long c(unsigned long long a) {return a&4294967295; } + +_c: + movl $4294967295, %ecx + movq %rdi, %rax + andq %rcx, %rax + ret + +//===---------------------------------------------------------------------===// -- 2.34.1