[llvm] r218170 - [X86] Erase some obsolete comments from README.txt
Robin Morisset
morisset at google.com
Fri Sep 19 16:56:46 PDT 2014
Author: morisset
Date: Fri Sep 19 18:56:46 2014
New Revision: 218170
URL: http://llvm.org/viewvc/llvm-project?rev=218170&view=rev
Log:
[X86] Erase some obsolete comments from README.txt
I just tried reproducing some of the optimization failures in README.txt in the
X86 backend, and many of them could not be reproduced. In general the entire
file appears quite bit-rotted, whatever interesting parts remain should be
moved to bugzilla, and the rest deleted. I did not spend the time to do that,
so I just deleted the few I tried reproducing which are obsolete, to save some
time to whoever will find the courage to do it.
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=218170&r1=218169&r2=218170&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Fri Sep 19 18:56:46 2014
@@ -2,17 +2,6 @@
// Random ideas for the X86 backend.
//===---------------------------------------------------------------------===//
-This should be one DIV/IDIV instruction, not a libcall:
-
-unsigned test(unsigned long long X, unsigned Y) {
- return X/Y;
-}
-
-This can be done trivially with a custom legalizer. What about overflow
-though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
-
-//===---------------------------------------------------------------------===//
-
Improvements to the multiply -> shift/add algorithm:
http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
@@ -83,43 +72,6 @@ It appears icc use push for parameter pa
//===---------------------------------------------------------------------===//
-This:
-
-void foo(void);
-void bar(int x, int *P) {
- x >>= 2;
- if (x)
- foo();
- *P = x;
-}
-
-compiles into:
-
- movq %rsi, %rbx
- movl %edi, %r14d
- sarl $2, %r14d
- testl %r14d, %r14d
- je LBB0_2
-
-Instead of doing an explicit test, we can use the flags off the sar. This
-occurs in a bigger testcase like this, which is pretty common:
-
-#include <vector>
-int test1(std::vector<int> &X) {
- int Sum = 0;
- for (long i = 0, e = X.size(); i != e; ++i)
- X[i] = 0;
- return Sum;
-}
-
-//===---------------------------------------------------------------------===//
-
-Only use inc/neg/not instructions on processors where they are faster than
-add/sub/xor. They are slower on the P4 due to only updating some processor
-flags.
-
-//===---------------------------------------------------------------------===//
-
The instruction selector sometimes misses folding a load into a compare. The
pattern is written as (cmp reg, (load p)). Because the compare isn't
commutative, it is not matched with the load on both sides. The dag combiner
@@ -303,42 +255,6 @@ opposed to two cycles for the movl+lea v
//===---------------------------------------------------------------------===//
-__builtin_ffs codegen is messy.
-
-int ffs_(unsigned X) { return __builtin_ffs(X); }
-
-llvm produces:
-ffs_:
- movl 4(%esp), %ecx
- bsfl %ecx, %eax
- movl $32, %edx
- cmove %edx, %eax
- incl %eax
- xorl %edx, %edx
- testl %ecx, %ecx
- cmove %edx, %eax
- ret
-
-vs gcc:
-
-_ffs_:
- movl $-1, %edx
- bsfl 4(%esp), %eax
- cmove %edx, %eax
- addl $1, %eax
- ret
-
-Another example of __builtin_ffs (use predsimplify to eliminate a select):
-
-int foo (unsigned long j) {
- if (j)
- return __builtin_ffs (j) - 1;
- else
- return 0;
-}
-
-//===---------------------------------------------------------------------===//
-
It appears gcc place string data with linkonce linkage in
.section __TEXT,__const_coal,coalesced instead of
.section __DATA,__const_coal,coalesced.
@@ -466,85 +382,6 @@ We should inline lrintf and probably oth
//===---------------------------------------------------------------------===//
-Use the FLAGS values from arithmetic instructions more. For example, compile:
-
-int add_zf(int *x, int y, int a, int b) {
- if ((*x += y) == 0)
- return a;
- else
- return b;
-}
-
-to:
- addl %esi, (%rdi)
- movl %edx, %eax
- cmovne %ecx, %eax
- ret
-instead of:
-
-_add_zf:
- addl (%rdi), %esi
- movl %esi, (%rdi)
- testl %esi, %esi
- cmove %edx, %ecx
- movl %ecx, %eax
- ret
-
-As another example, compile function f2 in test/CodeGen/X86/cmp-test.ll
-without a test instruction.
-
-//===---------------------------------------------------------------------===//
-
-These two functions have identical effects:
-
-unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return i;}
-unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;}
-
-We currently compile them to:
-
-_f:
- movl 4(%esp), %eax
- movl %eax, %ecx
- incl %ecx
- movl 8(%esp), %edx
- cmpl %edx, %ecx
- jne LBB1_2 #UnifiedReturnBlock
-LBB1_1: #cond_true
- addl $2, %eax
- ret
-LBB1_2: #UnifiedReturnBlock
- movl %ecx, %eax
- ret
-_f2:
- movl 4(%esp), %eax
- movl %eax, %ecx
- incl %ecx
- cmpl 8(%esp), %ecx
- sete %cl
- movzbl %cl, %ecx
- leal 1(%ecx,%eax), %eax
- ret
-
-both of which are inferior to GCC's:
-
-_f:
- movl 4(%esp), %edx
- leal 1(%edx), %eax
- addl $2, %edx
- cmpl 8(%esp), %eax
- cmove %edx, %eax
- ret
-_f2:
- movl 4(%esp), %eax
- addl $1, %eax
- xorl %edx, %edx
- cmpl 8(%esp), %eax
- sete %dl
- addl %edx, %eax
- ret
-
-//===---------------------------------------------------------------------===//
-
This code:
void test(int X) {
@@ -1398,20 +1235,6 @@ A similar code sequence works for divisi
//===---------------------------------------------------------------------===//
-These should compile to the same code, but the later codegen's to useless
-instructions on X86. This may be a trivial dag combine (GCC PR7061):
-
-struct s1 { unsigned char a, b; };
-unsigned long f1(struct s1 x) {
- return x.a + x.b;
-}
-struct s2 { unsigned a: 8, b: 8; };
-unsigned long f2(struct s2 x) {
- return x.a + x.b;
-}
-
-//===---------------------------------------------------------------------===//
-
We currently compile this:
define i32 @func1(i32 %v1, i32 %v2) nounwind {
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