cmovb %rcx, %rax
ret
-Seems like the jb branch has high likelyhood of being taken. It would have
+Seems like the jb branch has high likelihood of being taken. It would have
saved a few instructions.
//===---------------------------------------------------------------------===//
-Poor codegen:
-
-int X[2];
-int b;
-void test(void) {
- memset(X, b, 2*sizeof(X[0]));
-}
-
-llc:
- movq _b@GOTPCREL(%rip), %rax
- movzbq (%rax), %rax
- movq %rax, %rcx
- shlq $8, %rcx
- orq %rax, %rcx
- movq %rcx, %rax
- shlq $16, %rax
- orq %rcx, %rax
- movq %rax, %rcx
- shlq $32, %rcx
- movq _X@GOTPCREL(%rip), %rdx
- orq %rax, %rcx
- movq %rcx, (%rdx)
- ret
-
-gcc:
- movq _b@GOTPCREL(%rip), %rax
- movabsq $72340172838076673, %rdx
- movzbq (%rax), %rax
- imulq %rdx, %rax
- movq _X@GOTPCREL(%rip), %rdx
- movq %rax, (%rdx)
- ret
-
-And the codegen is even worse for the following
-(from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33103):
- void fill1(char *s, int a)
- {
- __builtin_memset(s, a, 15);
- }
-
-For this version, we duplicate the computation of the constant to store.
-
-//===---------------------------------------------------------------------===//
-
It's not possible to reference AH, BH, CH, and DH registers in an instruction
requiring REX prefix. However, divb and mulb both produce results in AH. If isel
emits a CopyFromReg which gets turned into a movb and that can be allocated a
//===---------------------------------------------------------------------===//
-Take the following C code
-(from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43640):
-
-struct u1
-{
- float x;
- float y;
-};
-
-float foo(struct u1 u)
-{
- return u.x + u.y;
-}
-
-Optimizes to the following IR:
-define float @foo(double %u.0) nounwind readnone {
-entry:
- %tmp8 = bitcast double %u.0 to i64 ; <i64> [#uses=2]
- %tmp6 = trunc i64 %tmp8 to i32 ; <i32> [#uses=1]
- %tmp7 = bitcast i32 %tmp6 to float ; <float> [#uses=1]
- %tmp2 = lshr i64 %tmp8, 32 ; <i64> [#uses=1]
- %tmp3 = trunc i64 %tmp2 to i32 ; <i32> [#uses=1]
- %tmp4 = bitcast i32 %tmp3 to float ; <float> [#uses=1]
- %0 = fadd float %tmp7, %tmp4 ; <float> [#uses=1]
- ret float %0
-}
-
-And current llvm-gcc/clang output:
- movd %xmm0, %rax
- movd %eax, %xmm1
- shrq $32, %rax
- movd %eax, %xmm0
- addss %xmm1, %xmm0
- ret
-
-We really shouldn't move the floats to RAX, only to immediately move them
-straight back to the XMM registers.
-
-There really isn't any good way to handle this purely in IR optimizers; it
-could possibly be handled by changing the output of the fronted, though. It
-would also be feasible to add a x86-specific DAGCombine to optimize the
-bitcast+trunc+(lshr+)bitcast combination.
-
-//===---------------------------------------------------------------------===//
-
Take the following code
(from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653):
extern unsigned long table[];
expensive addressing mode.
//===---------------------------------------------------------------------===//
+
+Consider the following (contrived testcase, but contains common factors):
+
+#include <stdarg.h>
+int test(int x, ...) {
+ int sum, i;
+ va_list l;
+ va_start(l, x);
+ for (i = 0; i < x; i++)
+ sum += va_arg(l, int);
+ va_end(l);
+ return sum;
+}
+
+Testcase given in C because fixing it will likely involve changing the IR
+generated for it. The primary issue with the result is that it doesn't do any
+of the optimizations which are possible if we know the address of a va_list
+in the current function is never taken:
+1. We shouldn't spill the XMM registers because we only call va_arg with "int".
+2. It would be nice if we could scalarrepl the va_list.
+3. Probably overkill, but it'd be cool if we could peel off the first five
+iterations of the loop.
+
+Other optimizations involving functions which use va_arg on floats which don't
+have the address of a va_list taken:
+1. Conversely to the above, we shouldn't spill general registers if we only
+ call va_arg on "double".
+2. If we know nothing more than 64 bits wide is read from the XMM registers,
+ we can change the spilling code to reduce the amount of stack used by half.
+
+//===---------------------------------------------------------------------===//
projects / oota-llvm.git / blobdiff