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mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2025-01-30 19:35:54 +00:00

add some notes on really poor codegen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41319 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2007-08-23 15:22:07 +00:00
parent aabd0359a3
commit 9e43d6316f

@ -1134,3 +1134,47 @@ pop into a dummy register instead of using addl/subl of esp. Just don't pop
into any return registers :)
//===---------------------------------------------------------------------===//
The X86 backend should fold (branch (or (setcc, setcc))) into multiple
branches. We generate really poor code for:
double testf(double a) {
return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0);
}
For example, the entry BB is:
_testf:
subl $20, %esp
pxor %xmm0, %xmm0
movsd 24(%esp), %xmm1
ucomisd %xmm0, %xmm1
setnp %al
sete %cl
testb %cl, %al
jne LBB1_5 # UnifiedReturnBlock
LBB1_1: # cond_true
it would be better to replace the last four instructions with:
jp LBB1_1
je LBB1_5
LBB1_1:
We also codegen the inner ?: into a diamond:
cvtss2sd LCPI1_0(%rip), %xmm2
cvtss2sd LCPI1_1(%rip), %xmm3
ucomisd %xmm1, %xmm0
ja LBB1_3 # cond_true
LBB1_2: # cond_true
movapd %xmm3, %xmm2
LBB1_3: # cond_true
movapd %xmm2, %xmm0
ret
We should sink the load into xmm3 into the LBB1_2 block. This should
be pretty easy, and will nuke all the copies.
//===---------------------------------------------------------------------===//