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[mips][FastISel] Apply only zero-extension to constants prior to their materialization.

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Summary:
Previously, we would sign-extend non-boolean negative constants and
zero-extend otherwise. This was problematic for PHI instructions with
negative values that had a type with bitwidth less than that of the
register used for materialization.

More specifically, ComputePHILiveOutRegInfo() assumes the constants
present in a PHI node are zero extended in their container and
afterwards deduces the known bits.

For example, previously we would materialize an i16 -4 with the
following instruction:

  addiu $r, $zero, -4

The register would end-up with the 32-bit 2's complement representation
of -4. However, ComputePHILiveOutRegInfo() would generate a constant
with the upper 16-bits set to zero. The SelectionDAG builder would use
that information to generate an AssertZero node that would remove any
subsequent trunc & zero_extend nodes.

In theory, we should modify ComputePHILiveOutRegInfo() to consult
target-specific hooks about the way they prefer to materialize the
given constants. However, git-blame reports that this specific code
has not been touched since 2011 and it seems to be working well for every
target so far.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11592

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243636 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Vasileios Kalintiris
2015-07-30 11:51:44 +00:00
parent 4b26be6066
commit 3b79f0ccc3
3 changed files with 15 additions and 19 deletions
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7
lib/Target/Mips/MipsFastISel.cpp
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@@ -290,12 +290,7 @@ unsigned MipsFastISel::materializeInt(const Constant *C, MVT VT) {
return 0;
const TargetRegisterClass *RC = &Mips::GPR32RegClass;
const ConstantInt *CI = cast<ConstantInt>(C);
int64_t Imm;
if ((VT != MVT::i1) && CI->isNegative())
Imm = CI->getSExtValue();
else
Imm = CI->getZExtValue();
return materialize32BitInt(Imm, RC);
return materialize32BitInt(CI->getZExtValue(), RC);
}
unsigned MipsFastISel::materialize32BitInt(int64_t Imm,