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Add more information to the EFLAGS note.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66515 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-03-10 00:26:23 +00:00
parent 1f2f31493e
commit 3328adda6b
1 changed files with 12 additions and 4 deletions
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16
lib/Target/X86/README.txt
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@ -1820,18 +1820,26 @@ LBB1_1: ## bb1
//===---------------------------------------------------------------------===//
Test instructions can be eliminated by using EFLAGS values from arithmetic
instructions. This is currently not done for and, or, xor, neg, shl, sra,
srl, shld, shrd, atomic ops, and others. It is also currently not done for
read-modify-write instructions.
instructions. This is currently not done for mul, and, or, xor, neg, shl,
sra, srl, shld, shrd, atomic ops, and others. It is also currently not done
for read-modify-write instructions. It is also current not done if the
OF or CF flags are needed.
The shift operators have the complication that when the shift count is
zero, EFLAGS is not set, so they can only subsume a test instruction if
the shift count is known to be non-zero.
the shift count is known to be non-zero. Also, using the EFLAGS value
from a shift is apparently very slow on some x86 implementations.
In read-modify-write instructions, the root node in the isel match is
the store, and isel has no way for the use of the EFLAGS result of the
arithmetic to be remapped to the new node.
Add and subtract instructions set OF on signed overflow and CF on unsiged
overflow, while test instructions always clear OF and CF. In order to
replace a test with an add or subtract in a situation where OF or CF is
needed, codegen must be able to prove that the operation cannot see
signed or unsigned overflow, respectively.
//===---------------------------------------------------------------------===//
test/CodeGen/X86/2009-03-07-FPConstSelect.ll compiles to: