Changes in directory llvm/lib/Target/X86:
README.txt updated: 1.28 -> 1.29
---
Log message:
add some performance notes from looking at sgefa
---
Diffs of the changes: (+34 -0)
README.txt | 34 ++++++++++++++++++++++++++++++++++
1 files changed, 34 insertions(+)
Index: llvm/lib/Target/X86/README.txt
diff -u llvm/lib/Target/X86/README.txt:1.28 llvm/lib/Target/X86/README.txt:1.29
--- llvm/lib/Target/X86/README.txt:1.28 Sun Jan 29 03:14:47 2006
+++ llvm/lib/Target/X86/README.txt Sun Jan 29 03:42:20 2006
@@ -248,3 +248,37 @@
instruction. With SSE it could be far better than this. Why is the sequence
above using 'setp'? It shouldn't care about nan's.
+//===---------------------------------------------------------------------===//
+
+Is there a better way to implement Y = -X (fneg) than the literal code:
+
+float %test(float %X) {
+ %Y = sub float -0.0, %X
+ ret float %Y
+}
+
+ movss LCPI1_0, %xmm0 ;; load -0.0
+ subss 8(%esp), %xmm0 ;; subtract
+
+//===---------------------------------------------------------------------===//
+
+None of the SSE instructions are handled in X86RegisterInfo::foldMemoryOperand,
+which prevents the spiller from folding spill code into the instructions.
+
+This leads to code like this:
+
+mov %eax, 8(%esp)
+cvtsi2sd %eax, %xmm0
+instead of:
+cvtsi2sd 8(%esp), %xmm0
+
+//===---------------------------------------------------------------------===//
+
+This instruction selector selects 'int X = 0' as 'mov Reg, 0' not 'xor Reg,Reg'
+This is bigger and slower.
+
+//===---------------------------------------------------------------------===//
+
+LSR should be turned on for the X86 backend and tuned to take advantage of its
+addressing modes.
+
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