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Diffstat (limited to 'contrib/llvm/lib/Target/PowerPC/README_ALTIVEC.txt')
| -rw-r--r-- | contrib/llvm/lib/Target/PowerPC/README_ALTIVEC.txt | 211 |
1 files changed, 211 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Target/PowerPC/README_ALTIVEC.txt b/contrib/llvm/lib/Target/PowerPC/README_ALTIVEC.txt new file mode 100644 index 0000000..1e4c6fb --- /dev/null +++ b/contrib/llvm/lib/Target/PowerPC/README_ALTIVEC.txt @@ -0,0 +1,211 @@ +//===- README_ALTIVEC.txt - Notes for improving Altivec code gen ----------===// + +Implement PPCInstrInfo::isLoadFromStackSlot/isStoreToStackSlot for vector +registers, to generate better spill code. + +//===----------------------------------------------------------------------===// + +The first should be a single lvx from the constant pool, the second should be +a xor/stvx: + +void foo(void) { + int x[8] __attribute__((aligned(128))) = { 1, 1, 1, 17, 1, 1, 1, 1 }; + bar (x); +} + +#include <string.h> +void foo(void) { + int x[8] __attribute__((aligned(128))); + memset (x, 0, sizeof (x)); + bar (x); +} + +//===----------------------------------------------------------------------===// + +Altivec: Codegen'ing MUL with vector FMADD should add -0.0, not 0.0: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8763 + +When -ffast-math is on, we can use 0.0. + +//===----------------------------------------------------------------------===// + + Consider this: + v4f32 Vector; + v4f32 Vector2 = { Vector.X, Vector.X, Vector.X, Vector.X }; + +Since we know that "Vector" is 16-byte aligned and we know the element offset +of ".X", we should change the load into a lve*x instruction, instead of doing +a load/store/lve*x sequence. + +//===----------------------------------------------------------------------===// + +For functions that use altivec AND have calls, we are VRSAVE'ing all call +clobbered regs. + +//===----------------------------------------------------------------------===// + +Implement passing vectors by value into calls and receiving them as arguments. + +//===----------------------------------------------------------------------===// + +GCC apparently tries to codegen { C1, C2, Variable, C3 } as a constant pool load +of C1/C2/C3, then a load and vperm of Variable. + +//===----------------------------------------------------------------------===// + +We need a way to teach tblgen that some operands of an intrinsic are required to +be constants. The verifier should enforce this constraint. + +//===----------------------------------------------------------------------===// + +We currently codegen SCALAR_TO_VECTOR as a store of the scalar to a 16-byte +aligned stack slot, followed by a load/vperm. We should probably just store it +to a scalar stack slot, then use lvsl/vperm to load it. If the value is already +in memory this is a big win. + +//===----------------------------------------------------------------------===// + +extract_vector_elt of an arbitrary constant vector can be done with the +following instructions: + +vTemp = vec_splat(v0,2); // 2 is the element the src is in. +vec_ste(&destloc,0,vTemp); + +We can do an arbitrary non-constant value by using lvsr/perm/ste. + +//===----------------------------------------------------------------------===// + +If we want to tie instruction selection into the scheduler, we can do some +constant formation with different instructions. For example, we can generate +"vsplti -1" with "vcmpequw R,R" and 1,1,1,1 with "vsubcuw R,R", and 0,0,0,0 with +"vsplti 0" or "vxor", each of which use different execution units, thus could +help scheduling. + +This is probably only reasonable for a post-pass scheduler. + +//===----------------------------------------------------------------------===// + +For this function: + +void test(vector float *A, vector float *B) { + vector float C = (vector float)vec_cmpeq(*A, *B); + if (!vec_any_eq(*A, *B)) + *B = (vector float){0,0,0,0}; + *A = C; +} + +we get the following basic block: + + ... + lvx v2, 0, r4 + lvx v3, 0, r3 + vcmpeqfp v4, v3, v2 + vcmpeqfp. v2, v3, v2 + bne cr6, LBB1_2 ; cond_next + +The vcmpeqfp/vcmpeqfp. instructions currently cannot be merged when the +vcmpeqfp. result is used by a branch. This can be improved. + +//===----------------------------------------------------------------------===// + +The code generated for this is truly aweful: + +vector float test(float a, float b) { + return (vector float){ 0.0, a, 0.0, 0.0}; +} + +LCPI1_0: ; float + .space 4 + .text + .globl _test + .align 4 +_test: + mfspr r2, 256 + oris r3, r2, 4096 + mtspr 256, r3 + lis r3, ha16(LCPI1_0) + addi r4, r1, -32 + stfs f1, -16(r1) + addi r5, r1, -16 + lfs f0, lo16(LCPI1_0)(r3) + stfs f0, -32(r1) + lvx v2, 0, r4 + lvx v3, 0, r5 + vmrghw v3, v3, v2 + vspltw v2, v2, 0 + vmrghw v2, v2, v3 + mtspr 256, r2 + blr + +//===----------------------------------------------------------------------===// + +int foo(vector float *x, vector float *y) { + if (vec_all_eq(*x,*y)) return 3245; + else return 12; +} + +A predicate compare being used in a select_cc should have the same peephole +applied to it as a predicate compare used by a br_cc. There should be no +mfcr here: + +_foo: + mfspr r2, 256 + oris r5, r2, 12288 + mtspr 256, r5 + li r5, 12 + li r6, 3245 + lvx v2, 0, r4 + lvx v3, 0, r3 + vcmpeqfp. v2, v3, v2 + mfcr r3, 2 + rlwinm r3, r3, 25, 31, 31 + cmpwi cr0, r3, 0 + bne cr0, LBB1_2 ; entry +LBB1_1: ; entry + mr r6, r5 +LBB1_2: ; entry + mr r3, r6 + mtspr 256, r2 + blr + +//===----------------------------------------------------------------------===// + +CodeGen/PowerPC/vec_constants.ll has an and operation that should be +codegen'd to andc. The issue is that the 'all ones' build vector is +SelectNodeTo'd a VSPLTISB instruction node before the and/xor is selected +which prevents the vnot pattern from matching. + + +//===----------------------------------------------------------------------===// + +An alternative to the store/store/load approach for illegal insert element +lowering would be: + +1. store element to any ol' slot +2. lvx the slot +3. lvsl 0; splat index; vcmpeq to generate a select mask +4. lvsl slot + x; vperm to rotate result into correct slot +5. vsel result together. + +//===----------------------------------------------------------------------===// + +Should codegen branches on vec_any/vec_all to avoid mfcr. Two examples: + +#include <altivec.h> + int f(vector float a, vector float b) + { + int aa = 0; + if (vec_all_ge(a, b)) + aa |= 0x1; + if (vec_any_ge(a,b)) + aa |= 0x2; + return aa; +} + +vector float f(vector float a, vector float b) { + if (vec_any_eq(a, b)) + return a; + else + return b; +} + |
