Author: lattner
Date: Thu Aug 23 10:22:07 2007
New Revision: 41319
URL: http://llvm.org/viewvc/llvm-project?rev=41319&view=rev
Log:
add some notes on really poor codegen.
Modified:
llvm/trunk/lib/Target/X86/README.txt
Modified: llvm/trunk/lib/Target/X86/README.txt
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README.txt?rev=41319&r1=41318&r2=41319&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Thu Aug 23 10:22:07 2007
@@ -1134,3 +1134,47 @@
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.
+
+//===---------------------------------------------------------------------===//
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