Add a note about integer multiplication by constants.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28551 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index d944cbf10392c796b3e0e3466ad2df995b4318b4..590a6dbebacba4463e7fa558cebb39c19b6f8767 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -622,3 +622,30 @@ How about implementing truncate / anyext as a property of machine instruction
 operand? i.e. Print as 32-bit super-class register / 16-bit sub-class register.
 Do this for the cases where a truncate / anyext is guaranteed to be eliminated.
 For IA32 that is truncate from 32 to 16 and anyext from 16 to 32.
+
+//===---------------------------------------------------------------------===//
+
+For this:
+
+int test(int a)
+{
+  return a * 3;
+}
+
+We currently emits
+       imull $3, 4(%esp), %eax
+
+Perhaps this is what we really should generate is? Is imull three or four
+cycles? Note: ICC generates this:
+       movl    4(%esp), %eax
+       leal    (%eax,%eax,2), %eax
+
+The current instruction priority is based on pattern complexity. The former is
+more "complex" because it folds a load so the latter will not be emitted.
+
+Perhaps we should use AddedComplexity to give LEA32r a higher priority? We
+should always try to match LEA first since the LEA matching code does some
+estimate to determine whether the match is profitable.
+
+However, if we care more about code size, then imull is better. It's two bytes
+shorter than movl + leal.