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| author | Cameron Zwarich <zwarich@apple.com> | 2011-02-21 00:22:02 +0000 |
|---|---|---|
| committer | Cameron Zwarich <zwarich@apple.com> | 2011-02-21 00:22:02 +0000 |
| commit | 8d7285d0e5eb5937a6682e884b883516377e903d (patch) | |
| tree | ccf0591dff9fa8874d00e480674cd7e77c84bbdf /lib/Target/X86/README.txt | |
| parent | 0b85d07d4611e7d704bf6550fbc624aff36a53b4 (diff) | |
| download | llvm-8d7285d0e5eb5937a6682e884b883516377e903d.tar.gz llvm-8d7285d0e5eb5937a6682e884b883516377e903d.tar.bz2 llvm-8d7285d0e5eb5937a6682e884b883516377e903d.tar.xz | |
The signed version of our "magic number" computation for the integer approximation
of a constant had a minor typo introduced when copying it from the book, which
caused it to favor negative approximations over positive approximations in many
cases. Positive approximations require fewer operations beyond the multiplication.
In the case of division by 3, we still generate code that is a single instruction
larger than GCC's code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126097 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Target/X86/README.txt')
| -rw-r--r-- | lib/Target/X86/README.txt | 10 |
1 files changed, 4 insertions, 6 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index c10e1709f6..ed3bff150b 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1888,12 +1888,10 @@ Compiles to: $clang t.c -S -o - -O3 -mkernel -fomit-frame-pointer _t: ## @t movslq %edi, %rax - imulq $-1431655765, %rax, %rcx ## imm = 0xFFFFFFFFAAAAAAAB - shrq $32, %rcx - addl %ecx, %eax - movl %eax, %ecx - shrl $31, %ecx - shrl %eax + imulq $1431655766, %rax, %rax ## imm = 0x55555556 + movq %rax, %rcx + shrq $63, %rcx + shrq $32, %rax addl %ecx, %eax movsbl %al, %eax ret |