Add an instcombine to clean up a common pattern produced
by the SRoA "promote to large integer" code, eliminating
some type conversions like this:
%94 = zext i16 %93 to i32 ; <i32> [#uses=2]
%96 = lshr i32 %94, 8 ; <i32> [#uses=1]
%101 = trunc i32 %96 to i8 ; <i8> [#uses=1]
This also unblocks other xforms from happening, now clang is able to compile:
struct S { float A, B, C, D; };
float foo(struct S A) { return A.A + A.B+A.C+A.D; }
into:
_foo: ## @foo
## BB#0: ## %entry
pshufd $1, %xmm0, %xmm2
addss %xmm0, %xmm2
movdqa %xmm1, %xmm3
addss %xmm2, %xmm3
pshufd $1, %xmm1, %xmm0
addss %xmm3, %xmm0
ret
on x86-64, instead of:
_foo: ## @foo
## BB#0: ## %entry
movd %xmm0, %rax
shrq $32, %rax
movd %eax, %xmm2
addss %xmm0, %xmm2
movapd %xmm1, %xmm3
addss %xmm2, %xmm3
movd %xmm1, %rax
shrq $32, %rax
movd %eax, %xmm0
addss %xmm3, %xmm0
ret
This seems pretty close to optimal to me, at least without
using horizontal adds. This also triggers in lots of other
code, including SPEC.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112278 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 82c3591..1372a18 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -454,6 +454,29 @@
Value *Zero = Constant::getNullValue(Src->getType());
return new ICmpInst(ICmpInst::ICMP_NE, Src, Zero);
}
+
+ // Transform trunc(lshr (zext A), Cst) to eliminate one type conversion.
+ Value *A = 0; ConstantInt *Cst = 0;
+ if (match(Src, m_LShr(m_ZExt(m_Value(A)), m_ConstantInt(Cst))) &&
+ Src->hasOneUse()) {
+ // We have three types to worry about here, the type of A, the source of
+ // the truncate (MidSize), and the destination of the truncate. We know that
+ // ASize < MidSize and MidSize > ResultSize, but don't know the relation
+ // between ASize and ResultSize.
+ unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+
+ // If the shift amount is larger than the size of A, then the result is
+ // known to be zero because all the input bits got shifted out.
+ if (Cst->getZExtValue() >= ASize)
+ return ReplaceInstUsesWith(CI, Constant::getNullValue(CI.getType()));
+
+ // Since we're doing an lshr and a zero extend, and know that the shift
+ // amount is smaller than ASize, it is always safe to do the shift in A's
+ // type, then zero extend or truncate to the result.
+ Value *Shift = Builder->CreateLShr(A, Cst->getZExtValue());
+ Shift->takeName(Src);
+ return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
+ }
return 0;
}